also: なんか
Indefinite pronoun referring to an unspecified thing. Used in questions, negative sentences, and conditional clauses.
Is there something to eat?
Please contact me if anything happens.
Adverbial use expressing that something is felt or perceived in an unclear way. Often used with ～となく or ～知らず.
I thought something was off.
I don't know why, but it went well.
what (do you mean)?; so what?
Only with reading なにか
Interjection used to question someone's vague or evasive statement. Restricted to the reading なにか.
"Hey..." "What?"
Would you like to eat something?
Today I feel somehow lonely.
Is there something to drink?
Is there some kind of problem?
May I help you with something?
何か is used for an unspecified thing in general, while どれか implies choosing one unspecified item from a known set of options.
何か simply means 'something' or 'what', while 何なのか probes for the deeper meaning or true identity of something.
何か is the everyday word for 'something/some', while 何らか is a more formal, often written equivalent used mainly as 何らかの or adverbially.
何か is the common, neutral word for 'something' usable in any register, while 何やら is more literary and adds a nuance of vagueness or indirectness.
何か means 'something' in a specific instance, while 何事 with も or でも means 'everything/anything' in a broader, more sweeping sense.
何か simply means 'something' in a straightforward way, whereas 何とか adds vagueness or a sense of 'or other' and can also mean 'somehow'.
何か means 'something' in affirmative sentences, while 何も requires a negative verb and means 'nothing'.
何か means 'something' (one unspecified thing), while 何でも means 'anything at all' (no restriction on which thing).