The Financial Times has an interesting story on language and children with fascinating facts.
The auxiliary system in English, including words such as can, should, must, be, have and do is notorious among grammarians for its complexity. There are about 24 billion logically possible combinations, such as "He might have eat" or "He did be eating" but only 100 are grammatical. Amazingly, one study showed that children made none of the possible errors. As Stephen Pinker observed in his book, The Language Instinct, this seems particularly astonishing when you consider that young children are notably incompetent at most other activities.It underscores the importance of interacting with children at the youngest of ages to stimulate later language development.