It's a little tricky because "class" can be both singular and plural, but in this case, it's referring to a specific group of words (like nouns, verbs, adjectives, etc.) so the plural is applied to the entire phrase.
It's a little tricky because "class" can be both singular and plural, but in this case, it's referring to a specific group of words (like nouns, verbs, adjectives, etc.) so the plural is applied to the entire phrase.