Learn Italian: Qualcuno ha scoperto qualcosa nel cassetto. - Someone discovered something in the drawer.

Qualcuno means "someone" or "anybody" and is only used for people. Qualcosa means "something" or "anything" and is only used for things. This sentence perfectly shows them together: someone (qualcuno) found a thing (qualcosa).

Does qualcuno always take a singular verb like ha?

Yes. Just like "someone" in English, qualcuno is always grammatically singular. Even if you don't know exactly who or how many people were involved, you always use the "he/she/it" form of the verb, which is ha here.

Why is the past tense of scoprire (to discover) written as scoperto?

The verb scoprire has an irregular past participle. Instead of following the normal -ire pattern to become "scoprito", it changes to scoperto. It follows the exact same pattern as aprire (to open), which becomes aperto.

Why does the sentence use nel cassetto instead of in il cassetto?