Cosa vs. qualcosa
While both cosa and qualcosa relate to the concept of a thing, they serve different grammatical functions and are not interchangeable. Cosa is a versatile word used both to ask what and to mean the noun thing, while qualcosa is an indefinite pronoun that specifically means something or anything.
Cosa
A1This word has two primary uses. First, as an interrogative pronoun, it is the most common way to ask what?. Second, as a noun, it means thing.
Cosa vuoi bere?
(What do you want to drink?)
Non so cosa fare questo weekend.
(I don't know what to do this weekend.)
Questa è una cosa molto importante.
(This is a very important thing.)
Ho messo tutte le cose nella valigia.
(I put all the things in the suitcase.)
La cosa che mi piace di più è la sua onestà.
(The thing I like most is his honesty.)
Qualcosa
A1This is an indefinite pronoun that means something or anything. It always refers to an unspecified, non-identified object, idea, or matter.
Vorrei mangiare qualcosa.
(I would like to eat something.)
C'è qualcosa che non va?
(Is there something wrong?)
Ho visto qualcosa di strano nel giardino.
(I saw something strange in the garden.)
Ti devo dire qualcosa di importante.
(I have to tell you something important.)
Ha detto qualcosa a bassa voce e non ho capito.
(He said something in a low voice and I didn't understand.)
Summary
In essence, the main difference lies in specificity. Use cosa when asking what? to identify something, or when referring to a specific thing (e.g., the thing you said). Use qualcosa to refer to an indefinite, unknown something or anything (e.g., I want to eat something). You ask cosa? to find out what qualcosa is.