Children who learn two languages from infancy generally reach the major language milestones, such as babbling, first words and first word combinations, at broadly the same ages as monolingual children, though their vocabulary is spread across two languages and each language develops according to how much and how richly they hear it. Children who begin a second language at preschool follow a different path, often with a quiet period and a stage of ready-made phrases before building their own sentences.
This article walks through bilingual development from about six months to six years, distinguishing simultaneous and sequential learners, and describes what a Vietnamese–English preschool can expect and do at each stage. It also notes the general signs that deserve a conversation with families and a professional opinion, while stressing that development varies widely and is never a basis for comparing or ranking children.
Simultaneous and sequential bilinguals
Researchers commonly distinguish between simultaneous bilingualism, in which a child hears two languages regularly from birth or very early, and sequential bilingualism, in which a child has established one language and then begins learning another, often when starting preschool. The line between them is not sharp, and many researchers use around age three as a rough boundary, but the distinction is useful for teachers.
Simultaneous bilinguals are building two first languages at once. Their development in each language reflects the input they receive: a child who hears Vietnamese from her mother and grandparents and English from her father may have more Vietnamese overall but strong English for the activities she shares with her father. Annick De Houwer's work on bilingual first language acquisition emphasises how closely each language tracks the child's experience.
Sequential bilinguals bring an existing language to school and add a new one. They often show the stages described in our article on the silent period: continued use of the home language, a nonverbal period, formulaic speech and then productive language. Many children in Vietnamese bilingual preschools are sequential learners of English, and international children are often sequential learners of both Vietnamese and English.
Six to eighteen months: listening, babbling and first words
Infants begin life able to distinguish speech sounds from many languages. In a classic 1984 study, Janet Werker and Richard Tees showed that infants growing up with English could distinguish certain non-English consonant contrasts at six to eight months, but that by around ten to twelve months most no longer did; their perception had tuned to the sounds of the language around them. Infants who hear two languages regularly tune to both.
Babbling typically appears in the second half of the first year, and first words usually arrive around the first birthday, with wide variation. Studies of bilingual infants, including work by Laura-Ann Petitto and colleagues, have generally found that bilingual babies reach these early milestones on a similar schedule to monolingual babies. Exposure to two languages does not delay the start of speech.
For a preschool caring for infants, the implications are about relationships and talk. Babies learn language from responsive adults who look at them, imitate their sounds, name what they are looking at and take turns in early "conversations" of coos and babble. In a bilingual infant room, caregivers can each use the language they speak most naturally, so that the babies hear warm, fluent speech in both, rather than halting speech in a language the caregiver finds difficult.
Eighteen months to three years: vocabulary growth and mixing
Many toddlers go through a period of rapid vocabulary growth in the second year and begin combining words into short phrases. Bilingual toddlers do the same, but their words are distributed across two languages. Some concepts they can name in both, creating pairs of translation equivalents such as "nước" and "water"; others they know in only one language, depending on where they met the concept.
An early influential model, proposed by Virginia Volterra and Traute Taeschner in 1978, suggested that young bilinguals begin with a single mixed vocabulary and only later separate their languages. Later research, notably by Fred Genesee from 1989 onwards, challenged this, showing that young bilingual children could use their languages differently with different people from an early age. The current consensus is that children differentiate their languages early, and that mixing reflects gaps in vocabulary, the mixing they hear around them, and growing skill rather than confusion.
In a toddler room, teachers respond to mixing by taking up the meaning and modelling naturally, offer lots of naming and narration in both languages, and keep routines predictable. They also share with families that mixing at this age is expected, which relieves a common anxiety.
Three to four years: sentences and choosing by listener
Between three and four, most children move from short phrases to longer sentences, ask many questions and begin to use language for pretend play, telling and explaining. Bilingual children increasingly choose their language according to the listener, speaking English to one teacher and Vietnamese to another, and may switch quickly when a new person joins a conversation.
This is also the age at which many children begin a second language at preschool and enter it as sequential learners. A class of three-year-olds may contain simultaneous bilinguals who are fluent in both school languages, Vietnamese children who are new to English, and international children who are new to both. Teachers need to read each child individually rather than treating the class as a single level.
Grammar in each language develops along its own lines. English learners at this stage commonly make overgeneralisation errors such as "goed" or "foots", which show that they are working out rules. Vietnamese learners work with classifiers, pronouns of address and tones. These developmental errors are signs of progress and are best met with recasts rather than correction.
Four to six years: stories, explanations and thinking about language
In the final preschool years, children's language becomes a tool for longer stories, explanations, arguments and plans. They can retell a story in sequence, describe how they built something, explain why they think the plant died and negotiate the rules of a game. In a bilingual child, these capacities may be stronger in one language than the other, usually the one in which she has had more rich conversation.
Metalinguistic awareness, the ability to think about language itself, grows noticeably in these years. Children may comment that a friend "talks funny", ask what a word means in the other language, invent rhymes or notice that a word sounds similar in two languages. Some studies have suggested that bilingual children develop certain aspects of metalinguistic awareness earlier, though findings vary with the task and the children studied.
For a preschool, this is the stage to deepen explanation and storytelling in both languages, to bring language comparisons into circle time, and to prepare children for primary school by building strong oral language, listening and sound awareness, particularly in Vietnamese for children heading into Vietnamese primary schools.
Input: the variable behind every stage
Across all these stages, one factor shapes bilingual development more than any other: the amount and quality of language a child hears and uses in each language. Studies of bilingual children consistently find that vocabulary and grammar in each language relate to how much exposure the child has to it, and that exposure through rich interaction with people matters more than background media.
Quality matters as much as quantity. Hearing a language from several different speakers, in conversations about many topics, in stories and in play, gives a child a richer model than hearing it from one source in one setting. For English in a Vietnamese preschool, this is a reason to spread English across many kinds of activity and several adults where possible, rather than concentrating it in a single daily lesson.
Input also has an emotional side. Children are more motivated to use a language that is spoken by people they love and in activities they enjoy. A language that appears only in instructions or tests is unlikely to flourish, however many hours it occupies. When teachers wonder why a child's English or Vietnamese is not growing, the first question to ask is what that child's experience of the language actually looks like across a day and a week.
A summary of stages and classroom responses
The table summarises typical patterns and the main classroom responses. Ages are approximate and individual variation is wide.
| Age | Typical bilingual patterns | How the classroom responds |
|---|---|---|
| 6–18 months | Tunes to sounds of both languages; babbling; first words around the first birthday | Responsive talk from caregivers in their fluent languages; naming and turn-taking |
| 18 months–3 years | Vocabulary across two languages; translation pairs; mixing | Narration and naming; predictable routines; recasting mixed speech |
| 3–4 years | Sentences; choosing language by listener; new sequential learners arrive | Individual reading of each child; chunks and sentence frames; patient waits |
| 4–6 years | Stories, explanations, metalinguistic awareness | Dialogic reading; sustained shared thinking; language comparisons |
A table like this is a map, not a checklist. A child who is early or late on one point is not a problem to solve, and any summary of bilingual development has to be read alongside how much and what kind of input the child receives in each language.
Signs that deserve a closer look
Most variation in bilingual development is normal, and bilingualism itself does not cause language disorders. Some patterns, however, should prompt a conversation with the family and possibly a professional assessment. These include little babbling or response to voices in infancy, no words in any language by the middle of the second year, very limited understanding of everyday language in the child's stronger language, loss of words or skills the child previously had, or difficulty being understood by family members in the home language well into the preschool years.
The key principle is that a bilingual child should be evaluated across both languages. A child who seems behind in English but is developing well in Vietnamese is likely simply learning English; a child who is behind in both may need support. Professionals who assess bilingual children ideally consider both languages and the child's history of exposure.
Teachers do not diagnose. Their role is to observe carefully, share concrete observations with families, ask about development at home and suggest that families consult a qualified professional when concerns persist. Advising families to drop a language because a child has difficulties is not supported by research and can cut a child off from important relationships.
Talking with families about milestones
Families often compare their child with cousins, neighbours or milestone lists from the internet, and bilingual children can seem behind when only one language is considered. Teachers can help by explaining, early and calmly, how bilingual development works: that vocabulary is spread across languages, that mixing is normal, that understanding usually runs ahead of speaking and that each language grows with the experience the child has in it.
Concrete observations are more reassuring than general statements. Telling a parent that her daughter followed a three-step instruction in English this week, or retold a story in Vietnamese with a clear beginning and end, gives the family something real to hold on to. Asking families what they notice at home, in each language, completes the picture and often reveals strengths the school had not seen.
When concerns do arise, the same partnership makes it easier to discuss them openly and decide together whether to seek a professional opinion.
Câu hỏi thường gặp
Do bilingual children start talking later than monolingual children?
Generally no. Research suggests bilingual infants reach early milestones such as babbling and first words on a similar schedule. Their vocabulary is spread across two languages, so counting one language alone can make them look behind when they are not.
What is the difference between simultaneous and sequential bilingualism?
Simultaneous bilinguals hear two languages regularly from birth or very early, while sequential bilinguals establish one language first and then learn another, often at preschool. Around age three is often used as a rough boundary between the two.
Is it normal for a bilingual toddler to mix languages?
Yes. Mixing is expected and reflects gaps in vocabulary, the mixing children hear around them and growing communicative skill. Research shows young bilinguals separate their languages early, so mixing is not a sign of confusion.
Should a child with a language delay stop learning a second language?
Research does not support dropping a language as a remedy. Bilingualism does not cause language disorders, and children with difficulties can become bilingual. Families should seek assessment from a professional who considers both languages and the child's exposure.
How should teachers assess bilingual development?
By observing language in use across both languages, noting understanding, participation, vocabulary and storytelling, and asking families about the home language. Observations inform planning and conversations with families; they are not turned into scores or used to compare children.
When should teachers raise a concern with families?
When a child shows little response to voices or her name, very limited understanding in her stronger language, loss of previous skills, or no progress over a long period in any language. Teachers share specific observations and suggest a qualified assessment.