Learn Italian: Queste notti sono molto difficili. - These nights are very difficult.

Notti is the plural of notte (night). Notte belongs to a group of nouns that end in -e in the singular and change to -i in the plural, regardless of their gender. Since notte is feminine, its plural notti is also feminine, which is why we use the feminine queste (these) instead of the masculine questi.

Why does the adjective difficili end in -i?

Difficili is the plural of difficile (difficult). Just like the noun notte, adjectives that end in -e in the singular change their ending to -i in the plural for both masculine and feminine words. Since notti is plural, we use difficili.

Why doesn't molto change to molte to match the feminine plural notti?

In this sentence, molto means 'very' and is acting as an adverb describing the adjective difficili (very difficult). When molto means 'very,' it never changes its ending. It only changes to molti, molta, or molte when it means 'many' or 'a lot of' and describes a noun directly.

How can I tell that a word like is feminine just by looking at it?