Grammar

So I'm curious what's the difference between open class and close class in grammar?

Open and closed class is a very basic classification of words.

The closed class group of words is the smaller of the two. These words are part of a group that will not expand (i.e. it is highly unlikely that the language will develop new words of this kind) and includes determiners (the, a, an...), prepositions (on, under, below...) and conjunctions (and, but..).

By contrast, the open class group of words can, and does, expand. These words include nouns, verbs, adjectives and adverbs. As society develops, new concepts are generated and so we have new nouns (internet, bitcoin...) and verbs (double-click, upload); adjectives and adverbs expand too, but less dramatically.

Pronouns (we, us, them, they, it...) are a subclass of the open class of nouns (which they represent) and are considered a closed class.