What is the difference between the two wordings and which one is correct?
(1) Why is past tense not used?
(2) Why is not past tense used?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Both are incorrect, but if you inserted the word "the", the first one would be correct, while the second would still be wrong:
(1) Why is the past tense not used? (correct)
Why is not the past tense used? (not correct)
"Not" modifies "used" rather than "the past tense": "Is the past tense used?" "No, the past tense is not used." Adding "Why" to the beginning of a sentence pulls the auxiliary verb into second position, but leaves "not" and the other words where they were.
UPDATE: As pointed out by @user3081485, you could also write:
(3) Why isn't the past tense used?
You know that we have the S-V inversion in most questions in English. In your examples the subject is (@Alan) the past tense, so we would invert that and is. "Why is the past tense not used?"
However, contractions have to stay together, so we'll invert "the past tense" with the whole contraction: "Why isn't the past tense used?"
See here for a bit more information Wikipedia on Contractions and Inversions.