I have been learning English and this question arose when I was doing some reading.
'not to do but to do' or ' not to do but do', which one is grammartically correct? And I have yet another question to ask: When do we omit 'to' in the infinitive? I wonder if there're some rules.
e.g. As in "the purpose is not to do A but to do B" and "whether to do A or to do B"
Thanks.
The word "to" acts as an subordinator, marking the infinitive clause. When two verbs are coordinated together, the subordinator is often only required on the first one.
So you can say "Its purpose is to cut wheat and harvest the grain". The subordinator "to" is not required on the second verb.
You can also use a contrasting coordination "Its purpose is to cut wheat but not harvest the grain". However when you negate the first verb "Its purpose is not to cut wheat but to harvest grain". The "to" helps the meaning but isn't actually essential.
I see "can you provide context? I can't answer until I see where it's being used" far too often. You shouldn't need context for this. The first example "not to do but to do" is correct. The rule is similar to adjectives in lists, ex. stagnant water, sour milk, and wet dogs, in that all words listed should have the same amount and nature of modifiers. If you're using verbs, either all of them or none of them should be preceded by "to." Example:
You can choose either to walk or to run, but you will not get there any faster.
