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)?

  • 5
    Even though the event happened in the past, the outcome (the reason why) is still true in the present. That is why you can say "So that is why I couldn't find my homework!" May 28, 2015 at 14:15
  • Okay cool. Thanks a lot, Kristina. This might be a dumb question, but what if it was a question rather than a statement? Say it was, "Is/Was that why I couldn't find my homework?" "Is that" would still work in this situation, correct?
    – Sinow
    May 28, 2015 at 14:22
  • 2
    Same thing..."Is" is acceptable and true, but really, there's nothing stopping you from using "was", either - because that would be true too. May 28, 2015 at 14:37

1 Answer 1


Both are used and both are acceptable, though you are most likely to hear "that's" rather than "that is/was".

However, "is" cannot be used if the reason, per se, happened in the past.

'Remember when you got an F?'

'Yes. That was because I couldn't find my homework because you hid it.'

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