Over the next several years, children gradually add in other pieces of the language, such as grammatical morphemes, and their sentences become longer and more complex. They begin, for example, to produce full questions and negative statements, as well as sentences with multiple clauses. Amazingly, by the time children are four or five years old, they have mastered most of the sounds and structural patterns of their language. If you know any kindergartners, you know that their language is quite developed!