Replacing a whole action, not a single word
Do so, do it and do that can replace an already mentioned action together with its objects and other dependent words. This makes the text shorter, but the listener must be able to recover easily what do refers to.
The substitute usually points back to a clear antecedent. If the preceding idea names several possible actions, it is better to repeat the intended verb or clarify the object than create a mysterious do so.
clearly named action → do / does / did / done + so / it / that
The council promised to publish the figures, and it did so in June.
The council promised to publish the figures, and it did so in June.
Did so replaces the whole phrase publish the figures; past did matches the completed action.
Mira labelled the boxes, and I did it the next morning.
Mira labelled the boxes, and I did it the next morning.
Did it refers to the specific task label the boxes, performed by a different participant.
The form of do fits the new clause
The substitute does not mechanically copy the tense from the first mention. Its form depends on the subject, tense and auxiliary construction in the new clause: do, does, did, doing, done; after a modal, use base do.
So, it and that do not change. Negatives and perfect forms are built around do: does not do so, did not do it, has done that.
does so; did it; would do that; has done so
Nora checks the log every day and does so before lunch.
Nora checks the log every day and does so before lunch.
The subject Nora in the Present Simple requires does so.
They did not repeat the test, although they had promised to do so.
They did not repeat the test, although they had promised to do so.
Did not repeat negates the past action. To do so picks up repeat the test after promised.
After to: leave to alone or add a complete substitute
When the repeated action would follow infinitive to, you can omit the verb and leave to itself: I felt the need to. You can also say to do so, to do it or to do that.
Bare to do is incomplete in this function: do needs so, it or that. This does not apply when do is an ordinary lexical verb with its own object, as in to do the work.
need to / need to do so / need to do it / need to do that; not need to do
I rarely write to newspapers, but that day I felt the need to.
I rarely write to newspapers, but that day I felt the need to.
To alone preserves the position of the omitted write to newspapers.
The editor wanted to correct the headline and decided to do so.
The editor wanted to correct the headline and decided to do so.
To do so is a complete formal substitute; to do without so would be incomplete here.
Do so: a neutral-to-formal link to an action
Do so is generally more formal than do it and do that. It is particularly natural in instructions, reports and explanations when the writer neutrally refers back to the same action or to a repeated series of actions.
This is a tendency, not a strict grammatical condition. Do so is possible with a different agent if the antecedent is clear; however, do it often sounds more natural for a specific task in everyday conversation.
named action + do so
Applicants must upload one file and should do so before Friday.
Applicants must upload one file and should do so before Friday.
Do so concisely refers back to the formal instruction upload one file.
The birds build nests on the north side; they do so because it is sheltered.
The birds build nests on the north side; they do so because it is sheltered.
Do so refers to a typical series of similar actions, not to a single object.
Do it: a specific task or a different agent
Do it often refers to a specific action with an object: reset the router, label the boxes, submit the form. It is particularly natural when another person performs the same action.
It behaves as a pronoun referring to an already known action. An unclear antecedent therefore also causes problems for do it: the listener must understand exactly what was done.
specific action with an object → do it
- object / O —
- Who or what an action affects: in I read a book, a book is the object.
Leo could reset the router, but I can do it now.
Leo could reset the router, but I can do it now.
Do it naturally transfers the specific task to a new agent.
Ana sorts the returns on Mondays, and Ben does it on Fridays.
Ana sorts the returns on Mondays, and Ben does it on Fridays.
Does it agrees with Ben and replaces sorts the returns.
Do that: an emphatic, deliberate or contrasting choice
Do that points more emphatically to the named action. It often appears when the speaker evaluates a deliberate act, rejects it, recommends it or contrasts it with another possibility.
That can create a sense of distance from the act, but the sentence and situation determine the attitude. Not every do that is negative, and do it or do so often remain grammatical alternatives with a different focus.
evaluated or contrasted action → do that
Would you share a client’s password? I would never do that.
Would you share a client’s password? I would never do that.
Do that emphatically rejects a deliberate act.
We could postpone the launch, but I would rather not do that.
We could postpone the launch, but I would rather not do that.
Do that refers to an option contrasted with another decision.
Overlap, register and neighbouring constructions
Do so, do it and do that are often interchangeable. It is useful to ask not which form is uniquely correct, but which focus fits: a formal link to an action, a specific task, or an emphatic attitude to an act.
Do not confuse these forms with I think so, where so replaces a whole proposition, or with lexical do in do the work. In some British contexts, do alone is possible after a modal or auxiliary, as in I should do; this is a related pattern and does not cancel the rule about incomplete to do.
In your own text, alternate substitution with precise repetition of the action. Too many pronoun references make a paragraph shorter, but not clearer.
form determines focus, not an absolute grammatical ban
He said he would cancel the booking and eventually did so.
He said he would cancel the booking and eventually did so.
Did so replaces cancel the booking. With an appropriate tone, did it or did that is also possible here.
The manager may approve the change; I think so.
The manager may approve the change; I think so.
I think so replaces the content of the proposition, rather than substituting for the verb phrase with do.
Common mistakes
Incorrect:I wanted to report the fault, so I decided to do.
Correct:I wanted to report the fault, so I decided to do so.
After to, bare do does not complete verb-phrase substitution; so makes the substitute complete.
Incorrect:The archive updates the file nightly and do so automatically.
Correct:The archive updates the file nightly and does so automatically.
The subject The archive requires does in the Present Simple.
Incorrect:The archive reviews the case daily and do so in public.
Correct:The archive reviews the case daily and does so in public.
In the Present Simple, singular the archive requires does so, just as it requires reviews in the first clause.
Incorrect:We asked Lee to leave, and he did so left immediately.
Correct:We asked Lee to leave, and he did so immediately.
Did so already replaces left; repeating left creates two verb heads.
Incorrect:You should do so it before Friday.
Correct:You should do it before Friday.
So and it are alternative complements of the substitute, not elements to combine.