“What” is “τι” or “αυτό που”?

Question

If I want to say “What”, should I say “τι” or “αυτό που”? Are they interchangeable?

When I am using an online translator, it will use both of these words for “what”.

For example, if I ask the translator to say “I know what you are saying”, the translator responds with “ξέρω τι λές”. If I ask the translator “what you said was wrong”, the translator responds with “Αυτό που είπες ήταν λάθος”.

When should I use each one, or are they interchangeable?

Answer 1

"Τι" is an interrogative pronoun which is used at the beginning of a direct or indirect question.  For this reason, the exact translation of "τί" is what.
For example:
"Τί κάνεις τώρα;" = "What are you doing now?" (direct question)
"Ξέρω τί κάνεις τώρα" = I know what are you doing now. (indirect question)

"Aυτό που" is a relative pronoun na dit means "that/which".  It refers to something that we have already said.
Sometimes we translate it as "what" but it is not interchangeable with "τί", especially when we ask something.  
For example:
"Το πιρούνι είναι αυτό που χρησιμοποιούμε για να φάμε"= "A fork is something that/which we use for eating" (here "αυτό που" refers to the "πιρούνι/fork")
"Αυτό που είπες ήταν λάθος" = "what you said was wrong"  (here "αυτό που" refers to something that we had previously said)
"Που είναι αυτό που αγόρασες;" = "Where is the thing that you have bought?" (here "αυτό που" refers to the thing that was bought)

It's wrong to say "Τί είπες ήταν λάθος" because it's not a direct or indirect question.