Learn Italian: La torta è davvero buona.

Buona describes the cake (la torta). Because torta is a feminine singular noun, the adjective must match it by ending in -a. If you were describing a masculine word, like a dessert (il dolce), you would use buono.

Can I use molto instead of davvero in this sentence?

Yes, but the meaning is slightly different. Molto buona means "very good," while davvero buona means "really good" or "truly good." Davvero is great for adding extra emphasis, like when a cake tastes surprisingly delicious.

Why do we use buona for "good" here, rather than bene?

Buona is an adjective, which means it describes a noun (a person, place, or thing, like our cake). Bene means "well" and is an adverb, which describes how an action is done (like speaking well or eating well). Since we are describing the cake itself and not an action, we must use buona.

Should davvero change its ending to davvera to match the feminine cake?

No. Davvero is an adverb, and adverbs in Italian never change their endings. They stay exactly the same regardless of whether the noun is masculine, feminine, singular, or plural. Only adjectives (like buona) change to match the noun.