Meaning of different responses to a negative imperative

Replying to negative imperatives in general

Yes, you can answer in both ways, but the two answers likely (though not always) mean diametrically opposite things.

Whichever way you reply, you’re basically repeating the entire sentence; the auxiliary used to create the future construction (will) just takes the place of the auxiliary used to create the imperative (do). Then the bits of the sentence that are the same (forget to buy milk) are left out, because we usually leave out repeated information from sentences if we can.

With some types of negative imperatives, such as this one in “don’t forget to…”, it is possible to consider the auxiliary in the reply (will) not just a substitution for the auxiliary do in the imperative, but instead for as much of the verb phrase as makes sense. In this case, that would mean do [not] forget to buy. This of course creates a certain amount of ambiguity, since both will/won’t will refer either to [not] buying milk or [not] forgetting to buy milk—exact opposites, in other words.

Which of the two possible interpretations is intended will usually be clear from the context—especially from the tone of voice and the body language the answerer uses.

This ambiguity is not possible with all negative imperatives, because often the auxiliary do is as much of the verb phrase as can be substituted and still make sense. Consider:

— Don’t leave me!
— I will!

There is nothing more after do that can be replaced by will here, so “I will” has to be equivalent to “I will [leave you]”—it has no other option. The main verb, leave, can’t be included in the substitution, of course, because that would yield the ungrammatical underlying sentence “*I will you”; whereas in “Don’t forget to buy milk”, ‘expanding’ the substitution still works, because “I will [forget to buy milk]” and “I will [buy milk]” are equally grammatical and both make sense in the context. The main verb buy is of course akin to leave: “I will [milk]” is ungrammatical.

Buying or forgetting milk

— Don’t forget to buy milk!
— I won’t.

With neutral intonation and body language, you’re saying here that you won’t forget to buy milk; the repeated bits left out are basically “forget to buy milk”. If, on the other hand, you square your jaw and say in a defiant way, “I won’t!!!”, you could actually be stubbornly refusing to buy milk.

In this case, the first scenario will nearly always be the intended one unless there are definite context clues to indicate otherwise—the second scenario is fairly rare overall in comparison.

— Don’t forget to buy milk!
— I will!

If you square your jaw and defiantly say, “I will”, you’re almost certainly saying that you will forget to buy milk, and (as Robusto says), you’re being a bit confrontational about it. “I don’t care if you tell me not to forget to buy milk, I’ll damn well forget to buy milk if I want to!”. That’s relatively unambiguous.

But with neutral intonation and body language, it’s a bit more ambiguous. You’re most likely saying that you will buy milk (just because that’s the most likely reply to an imperative like that); the repeated bits left out are then just “buy milk”.

But there is still a fair chance that you’re actually saying “I will forget to buy milk”, and a particular intonation and look can make this the more likely explanation (imagine a resigned voice and a bit of rolling your eyes at your own forgetfulness: “Yeah, you say that now, but I’ll bet once I get to the store I’ll forget the bloody milk anyway…”).