Grammar can be thought of as the set of rules that help us to form new words out of existing words (e.g., we may not know what 'greps' means, but we might guess that it's the plural of 'grep'). More specifically, this type of 'within word' grammar is called 'morphology' by linguists. Grammatical rules also constrain the way words are put together to form sentences. For example, we say "The teacher sees the students" and not (in standard English) "The teacher see the students".

	Grammar has a creative component (especially in poetry) but there are still rules that need to be mastered in order to understand words, phrases and sentences.
	Vocabulary and grammar are the first steps toward comprehension, but they are not the whole story in comprehension.
	We use grammar a bit differently in different social situations (e.g., talking with a close friend vs. giving a formal presentation), so if you hear what sound like grammatical errors to you, these are probably things the child has heard from others. This is a good opportunity to talk about different ways to use language.