Here's an example of what's puzzling me (I'll use a brother/sister arguing example):
SISTER: I couldn't find my homework, so I got an F on the assignment.
BROTHER: You couldn't find your homework because I hid it from you.
SISTER: So that is/was why I couldn't find my homework!
"Couldn't" is past tense, correct? If so, "that was" would seem to be the correct choice to me just because I was told you normally aren't supposed to mix tenses. However, "that is" just sounds correct to me even though it's present tense. So which is correct in this situation (or any other situation that might be similar to this example)?