0–4 months

Can tell the difference between speech sounds (phonemes). Cooing, especially in response to speech.

4–6 months

Babbles consonants.

6–10 months

Understands some words and simple requests.

10–12 months

Begins to use single words.

12–18 months

Vocabulary of 30–50 words (simple nouns, adjectives, and action words).

18–24 months

Two-word phrases of about 1,000 words; production of phrases and incomplete sentences.

24–36 months

Vocabulary of about 1,000 words; production of phrases and incomplete sentences.

36–60 months

Vocabulary grows to more than 10,000 words; production of full sentences, mastery of grammatical morphemes (such as –ed for past tense) and function words (such as the, and, but). Can form questions and negations.