Imagine for a moment what it would be like to have words, but no systematic way to combine them. You'd have to communicate in tiny one-word installments – “thirsty,” “water,” “give.” That must be roughly what it's like to be a fifteen-month-old child with something to say.
There's considerable incentive, then, for children to learn how to create sentences, and it doesn't take them long to get started, as we'll see. The development of children's sentence-building skills can be roughly divided into two phases.
The first phase, which begins around the age of eighteen months, sees the appearance of relatively simple two- and three-word patterns. These early sentences are primitive and often incomplete, but they mark the start of something big.
During the second phase, which begins around age two or so, the missing pieces are filled in and there is rapid growth in the ability to produce a wide variety of complex constructions.
The basic recipe for sentence building is simple: Combine two words with the right fit (say, an adjective and a noun, or a noun and a verb) in the right order. Repeat the process as many times as necessary, adding one new word or combination of words each time.
Take, for example, the sentence The glass broke. You start with the words the and glass, and combine them to create the phrase the glass.