English — Ellipsis and substitution
To avoid repeating words, English either leaves them out (ellipsis) or replaces them with a short pro-form. In I can't but you can, the second clause drops the repeated verb; in I think so and I hope not, so and not stand in for a whole clause.
How it works
After an auxiliary or modal, drop everything that repeats: She has finished, but I haven't (— finished). Use so to stand for a positive clause after think, hope, believe, guess (I think so), and not for the negative one (I hope not). Replace a countable noun with one/ones (the red one, the small ones), and a whole action with do so or do it (He asked me to leave, so I did so).
Examples
| Example | What is left out / replaced |
|---|---|
| I can't, but you can. | Verbal ellipsis after a modal (can leave, not just can) |
| She has, but I haven't. | Verb phrase dropped after the auxiliary have |
| Is it raining? — I think so. | so replaces the positive clause it is raining |
| Will it rain? — I hope not. | not replaces the negative clause it will not rain |
| Not this cup — the blue one. | one replaces the countable noun cup |
| I prefer the fresh ones. | ones = plural of one, for a countable noun |
| She was told to wait, and she did so. | do so replaces the whole action wait |
Good to know
Do not say I think it before think — the pro-form is so, so use I think so, not I think it. Likewise, one/ones works only for countable nouns: for uncountables, repeat the auxiliary or use ellipsis instead — Do you want tea? — Yes, please (not Yes, I want one). And do so is more formal and usually needs a deliberate action; for everyday spoken English, do it or just the bare auxiliary (I did) is more natural.