For direct questions:
* τί (ti): This is the most common and general form.
* ποιο (poio): This is used for asking about a specific thing from a group or a choice.
* τις (tis): This is used for asking about a person.
For indirect questions:
* τι (ti) is used for asking about something in general.
* ποιο (poio) is used for asking about a specific thing.
Examples:
* Τι θέλεις να φας; (What do you want to eat?)
* Ποιο αυτοκίνητο σου αρέσει; (Which car do you like?)
* Τις ξέρεις; (Who do you know?)
* Δεν ξέρω τι να κάνω. (I don't know what to do.)
* Δεν ξέρω ποιο να διαλέξω. (I don't know which one to choose.)
It's important to choose the correct form of "what" based on the context of your question.