Girlfriend is a noun.
It is a verb.
The word 'in' would be a preposition in the example sentence you gave.
Adverb
How to use the word in a sentence.
onomatopoeia
it depends what word it is it could be any part of speech depending on the sentence
What "part of speech" is the word "said?"
In a sentence, the word "action" can function as a noun.
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