Study materials
Two sentences with the same content. The second is three words shorter, and that is how people speak.
I haven't finished, but she has finished.
I haven't finished, but she has.
English removes whatever the listener can restore. The only question is where the limit of recoverability lies.
Only the auxiliary remains
Once a verb phrase has been named, there is no need to repeat it. The word carrying the grammar stays: a form of be, have, do or a modal.
I haven't finished, but she has.
He can swim faster than I can.
There is one condition: the forms in both halves must be compatible. If the first half has was and the second were, no single auxiliary will fit both.
Do so, do it, do that
These three phrases replace a whole action, and they differ by register and by how exactly the action is repeated.
| Form | Register | What it replaces |
|---|---|---|
| do so | formal, written | the same action by the same doer |
| do it | neutral | a specific action |
| do that | colloquial | what was just named, pointing back |
Applicants must sign the form. Failure to do so delays the process.
An important limit: do so replaces an action, not a state. The sentence She knows the answer, and I do so too does not work.
Where it breaks down
The first failure: the forms in the two halves do not match.
She was late and they too. → She was late and so were they.
The second: do so used for a state.
She knows the answer, and I do so too. → and I do too.
The third: the wrong auxiliary.
She has arrived. So do I. → So have I.
All three are caught by one test. Restore the missing part in full in your head. If it does not match the first half, the omission is wrong.
Try it right now
Take three pairs of sentences from your correspondence and cut the repetition in each with an auxiliary.
Then check each: does the restoration give back exactly what was in the first half?