As children learn to speak in sentences, they also begin to use basic forms of correct grammar. Although children may not get every part of a sentence correct, they are usually following grammatical rules. This includes learning to change a simple statement (“mommy shoe”) into a negative statement (“not mommy shoe”). Between 2 and 2½ years, most children begin to speak in increasingly complex sentences that are grammatically correct (or very close). For example, children learn to ask questions (“Where birdie go?”) and as they get closer to age 3 many will create proper negative sentences (“I won’t wear a mitten”).
De Villiers, P. A., & de Villiers, J. G. (1992). Language development. In Bornstein, M. H., & Lamb, M. E. (Eds.), Developmental psychology: An advanced textbook. Hillsdale, NJ: Erlbaum.