Between ages 2-6, children's vocabulary grows enormously from 200 words to 10,000 words. During this period, children learn new words through fast mapping and contrasting new words with words they already know. They also start making guesses about meanings and developing biases like mutual exclusivity around ages 2-3. Adults play an important role by recasting language, expanding on what children say, listening attentively, and encouraging further discussion. By ages 3-4, children's grammar develops to include simple sentences following subject-verb-object order and mastery of basic rules and structures, though questions and agreement may still vary. Language acquisition is supported through conversations, reading books, and outings that stimulate learning.