Hi, 1- It's time to start revising for the exam. You haven't revised yet. (Doesn't mean that you haven't studied yet.
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imantaghavi1- It's time you started revising for the exam. (the revising hasn't been done, but should have been done, right?)Correct. The speaker really wants you to start revising now. May be he/she is worried about you writing some exam.
imantaghavi2-It's time we set off. The train leaves in half an hour. (what does this