Learn Italian: Questi libri sono molto difficili. - These books are very difficult.

Questi is the masculine plural form of questo (this). Because libri (books) is masculine and plural, we must use questi to mean "these". If it were just one book, we would say questo libro.

Doesn't sono mean "I am"? Why is it used for "these books"?

Sono can mean both "I am" (io sono) and "they are" (loro sono). Here, questi libri (these books) takes the place of "they", so sono translates to "are". You can tell which meaning is intended by looking at the subject of the sentence.

Why doesn't molto change to molti to match the plural libri?

In this sentence, molto means "very" and is describing how difficult the books are. When molto is used to mean "very", it acts as an adverb and never changes its ending. It only changes to match the noun when it means "many" or "a lot of" (for example, molti libri means "many books").

I see that difficili ends in -i. What is its singular form?