Babies understand grammar long before they learn how to speak

That's the finding of Jill Lany, a psychologist at Notre Dame. She found that babies as young as 12 months can identify what she calls "adjacent relationships", which means they can tell what part of speech a word is by what words or phrases are near it. These sorts of patterns provide a sort of basic grammatical structure for the infant's language acquisition later on. Lany provides an example of what she means:

"If I were to say to you, 'Oh look, it's a dax,' you might not know what a 'dax' is, but the cue 'it's a' lets a baby know that what follows is an object. We often think about grammar coming after word-learning, but in fact, my research shows that all this information that babies are picking up in that first year of life about how words are occurring in their language, actually is supporting this process of word-learning prior to mastery of language."

Lany says a baby will be able to distinguish between how different words are used by speakers well before they actually grasp their specific meanings. While "It's a dax" indicates a noun (and possibly that you're starting your baby on Deep Space Nine at a very early age), something like "I'm daxing it" would indicate a verb. Her research suggests babies can pick up on that within a year, and by 15 months they can track much more complicated, "non-adjacent" relationships, in which the word cue is much further from the word it describes. Lany concludes:

"Babies are constantly looking for language clues in context and sound. My research suggests that there are some surprising clues in the sound stream that may help babies learn the meanings of words. They can distinguish different kinds of words like nouns and verbs by information in that sound stream."

http://io9.com/5867029/babies-understand-grammar-long-before-they-learn-how-to-speak