Learn Italian: La quinta lezione è molto difficile. - The fifth lesson is very difficult.

Ordinal numbers like quinto (fifth) act like adjectives in Italian, meaning they must change their ending to match the noun they describe. Because lezione is a feminine singular noun, the ending changes to -a, making it quinta.

Lezione ends in -e, so how do we know it is a feminine noun?

While nouns ending in -e can be either masculine or feminine, words that end in -zione are almost always feminine in Italian. Because it is feminine, it requires the feminine article la and the feminine ordinal quinta.

Should molto change to molta to match the feminine noun?

Not in this sentence. When molto means 'very' and describes an adjective (like difficile), it acts as an adverb and its ending never changes. It only changes its ending when it means 'a lot of' and is attached directly to a noun (like molta pasta).

Why doesn't the adjective difficile end in -a to match the feminine noun lezione?