How many kinds of words are there in English?

There are 8 kinds of Words in English:

1. Nouns
   
2. Verbs
   
3. Pronouns
   
4. Adjectives
5. Adverbs
   
6. Prepositions
7. Conjunctions
8. Interjections

Some Authorities include a 9th category, called Pro-verbs. (Example: "Did you eat? No, I didn't." Didn't substitutes for "did not eat."