"I just don't really understand word classes"

There are 7 main word classes, so let’s focus on those for now. They are: Nouns, Pronouns, Verbs, Adverbs, Adjectives, Prepositions and Determiners. You can work out which class a word belongs to using both grammatical clues and semantic clues (the word’s meaning). For example, we know “pig” is a noun from both semantic clues and grammatical clues. The semantics of the word pig can help tell us it’s a noun because we know that a pig is an animal, an animal is a “thing” and a noun is typically known as a “person, place or thing”. The grammatical clues that “pig” is a noun are as follows: “pig” can be used with a determiner “a pig” or “the pig” and “pig” can take a suffix “-s” to show plural “pigs”.