Some writers say that when the question asks for a subject answer (as in Which form is the correct one? - This form is the correct one) rather than a complement answer (What's the time? - The time is eight o'clock), then there are two possible word orders:
Which form is correct?
1. Can you tell me which is the correct form?
2. Can you tell me which the correct form is.
If you are a learner, the safest 'rules' are those suggested by Soup and me. Just be aware that you may encounter sentences, such as #1 which do not follow the rules. Don't worry - #2 is acceptable.