English grammar - Roger Berry 2012

B1.2 Open and closed word classes
B1 Word classes
Section B Development

This refers to an important distinction between types of word class. Open word classes are those that are open to new members; closed classes are not. For example, it is relatively easy to make a new noun to describe some invention or new behaviour, for example chocoholic, but the same is not true of pronouns. Here is a list of the two types, as identified for modern English:

□ open: noun, adjective, adverb, (main) verb

□ closed: pronoun, determiner, preposition, auxiliary, conjunction

The division into main verb and auxiliary (verb) is made precisely because of this open/closed distinction.

There are other differences between open and closed classes. Words in open classes tend to be longer and less frequent than those in closed word classes; they also supply more of the meaning or content in language. Closed words classes carry less meaning and often supply the function words that are mentioned by Swan in the reading in D1. For example, when we say the capture of the murderer, the word of does not have any content meaning; it merely shows that the following noun, murderer, can be considered the object of capture.

It is not entirely true to say that the closed classes do not admit new members; for example you guys could be added to the class of pronouns (as an informal plural version of you - see B2). But the process is much slower.