Which parts of speech are open and closed word classes?This is related to Uniiversal Grammar.
There are seven word classes in the English language. These word classes are Nouns, Verbs, Adjectives, Adverbs, Prepositions, Conjunctions, and Determiners. Some of these classes gain (and lose) lexemes (words) on a regular basis. Some of these classes do not gain (or lose) lexemes at all, the class is fixed. The classes that gain and/or lose lexemes are Open Classes, while the classes that are fixed and neither gain nor lose lexemes are called Closed Classes.
The two largest open word classes are also the two largest word classes, these being nouns and verbs. Nouns are potentially unlimited with seemingly infinite possibilities for new nouns. By definition, nouns are therefore an open word class. The same is true for verbs although verbs may not have the infinite possibilities that nouns do. Since new verbs are being added continually, like the new verb texting ("I'm texting my mother."), verbs are also an open class.
The three closed word classes are prepositions, determiners, and conjunctions. No new lexemes in these classes have been invented. The number of lexemes in each of these classes is fixed and limited by the exclusion of new ones, therefore by definition they are closed classes.
The other two open word classes are adjectives and adverbs, however, their possibilities for expansion are much more limited than the two major open word classes (nouns and verbs). The adjective word class can be expanded through adjective forming suffixes like -able, -ish, -ous, -some, -ive, -ent, and -esque, etc. The adverb word class can be expanded through adverb forming suffixes, but not to the same extent as adjectives. Common adverb forming suffixes are -ly and -ward/-wards.