What is the ninth part of speech?

There is no "ninth part of speech" in traditional grammar. The eight traditional parts of speech are:

1. Nouns (person, place, thing, or idea)

2. Pronouns (words that replace nouns)

3. Adjectives (words that describe nouns)

4. Verbs (words that show action or state of being)

5. Adverbs (words that describe verbs, adjectives, or other adverbs)

6. Prepositions (words that show the relationship between a noun or pronoun and another word in the sentence)

7. Conjunctions (words that connect words, phrases, or clauses)

8. Interjections (words that express strong emotion)

While some grammar systems may identify additional categories or have more nuanced classifications, the eight traditional parts of speech form the foundation of English grammar.