'Your' is second person possessive adjective in a sentence.
it depends what word it is it could be any part of speech depending on the sentence
The word sentences is a noun. It is the plural form of sentence.
She stopped her speech in mid-sentence to answer the question.
What "part of speech" is the word "said?"
it is an English speech
The individual word "sentence" is a common noun. However, a sentence itself is comprised of many different parts of speech.
The president's speech was particularly moving today.
The term for knowing a word's part of speech in order to appropriately use it in a sentence is "word-usage."
By use of the word in a sentence.
The word 'in' would be a preposition in the example sentence you gave.
The use of the word in the sentence derminates the part of speech
The word 'this" in the sentence is an adjective describing book.