it depends what word it is it could be any part of speech depending on the sentence
it is an English speech
A suffix changes a word's part of speech. For example, the word 'happy' is an adjective. But when you add a suffix, which is an ending, it can change the part of speech. Happily is an adverb. Happiness is a noun.
verb
Noun or adjective. Depended on the sentence.
An active verb is required; any other part of speech may be present.
What "part of speech" is the word "said?"
The action in a sentence is the part of speech known as a verb.
The term for knowing a word's part of speech in order to appropriately use it in a sentence is "word-usage."
It is a verb.
By use of the word in a sentence.
A word is a part of speech, not a sentence like the one in your example.
The use of the word in the sentence derminates the part of speech
The word 'this" in the sentence is an adjective describing book.
it is an English speech
The word 'in' would be a preposition in the example sentence you gave.
Adverb
Girlfriend is a noun.