Which one is correct ?

  1. "I don't know what was that"


  1. "I don't know what that was"

The second sentence is the correct word order. You may hear the first sentence spoken in conversation but it will be punctuated like "I don't know. What was that?"

  • Just a note to point out that the meaning of "I don't know. What was that?" is not especially close to the meaning of "I don't know what that was". – GreenAsJade May 30 '14 at 0:42

The correct answer, as Pavja2 said, is the second one. It's called an "indirect question". They aren't normal questions because they have the same word order as statements.

More examples:

  • I don't know where John is.
  • I wonder if he will come to the party.
  • Do you know where he went after school?