To avoid repeating words, English replaces them with a short word (substitution) or leaves them out (ellipsis). Common substitutes are so, not, do so, one and ones, and the auxiliary verb.
After verbs such as think, hope, believe, expect and suppose, use so instead of a whole clause.
For a negative idea, the pattern depends on the verb. Use not after hope, guess and be afraid: I hope not, I guess not, I'm afraid not. With think, the usual negative in everyday speech is I don't think so (I think not is very formal and rare).
Use do so to avoid repeating a verb phrase. It is formal.
Use one or ones for a countable noun already mentioned. See One and ones: a smaller one, the ones by the door.
Use an auxiliary verb to avoid repeating the whole verb phrase. This happens in short answers and after and, but or than.
When two clauses share the same subject or verb, leave it out of the second clause after and, but and or.
You cannot drop a repeated subject like this after linkers such as before, after, because, when and while (not: We watch TV after have dinner). But before, after, when and while can be followed by an -ing form instead: We watch TV after having dinner. / While waiting for the bus, she read a book.
In informal speech, people leave out words at the start of a sentence when the meaning is clear.
✗I hope it.
✓I hope so.
In this short reply, so replaces the full that-clause (that he will be there); after hope, it does not work as that substitute.
✗Do you like coffee? Yes, I like.
✓Do you like coffee? Yes, I do.
In a short answer, use the auxiliary (do), not the main verb - Yes, I like has no object and sounds incomplete.
✗My old bike broke, so I bought new one.
✓My old bike broke, so I bought a new one.
One replaces the noun bike, but it still needs a: a new one, not new one.
See One and ones: a smaller one, the ones by the door for one and ones, Short answers, so do I, neither do I, and I think so for short answers, and Discourse markers for spoken linking words.
Eight questions. Choose one answer for each.