When the rest is already clear, you can stop after the helping word (be, have, do) or a word like can, will, might. You don't repeat the verb everyone already knows.
Stop after the helping word
Are you coming? → I might come. → I might.
Have you finished? → No, I haven't finished. → No, I haven't.
She said she would call, and she did call. → ... and she did.
Try it: A: Can you help me later? B: ... Answer in the short, natural way. Stop after the helping word. (Leave out the word you don't need.) Answer: I'm afraid I can't. — Everyone already knows the verb (help), so stop after the helping word: I'm afraid I can't. Adding "help you" repeats what's already clear, so leave it out. The short answer is more natural here.
Use does / did instead of repeating
To avoid repeating a verb you just used, swap in do / does / did. This is common when you compare two people.
She works harder than he works. → She works harder than he does.
I enjoyed it more than my friend enjoyed it. → I enjoyed it more than my friend did.
He plays better than I play. → He plays better than I do.
Not "She works harder than he works." — say "She works harder than he does.". Swap the repeated verb for does: than he does, not "than he works".
Try it: Lars drives more carefully than Diego ... Finish without repeating the verb. (Leave out the word you don't need.) Answer: Lars drives more carefully than Diego does. — To avoid saying "drives" twice, swap in does: than Diego does. Repeating "drives" sounds heavy when does already stands for it, so leave it out. Does carries the same verb without the repetition.
so and not stand in for a whole idea
After verbs like think, hope, believe, expect, suppose, be afraid, replace a whole repeated idea with so (yes side) or not (no side). With think and believe, the no side is usually "I don't think so" rather than "I think not".
Will they win? — I hope they win. → I hope so. / I hope they don't win. → I hope not.
Is the shop open? — I believe it's open. → I believe so.
Are we late? — I'm afraid we're late. → I'm afraid so. / I don't think we're late. → I don't think so.
Not "Is she coming? — I hope she isn't coming." — say "Is she coming? — I hope not.". Use not to stand in for the whole idea: I hope not, not the full repeated sentence.
Try it: A: Is she coming? B: ... Answer without repeating the whole idea. She probably isn't coming. (Leave out the word you don't need.) Answer: I don't think so. — After think, swap the whole idea for so: I don't think so. Repeating "she is coming" undoes the point of the short form, so leave it out. So stands in for the whole clause.