Our textbook says,

A: "I have to go shopping this afternoon."
B: "Don't forget the bread."
A: "No, I won't."

About the last sentence, why can't you say "No, I don't."?

i.e. Why is "No, I don't." not acceptable in this case:

  • 3
    Oddly, if you are a someone that does not have the ability to forget, "I don't" would be acceptable because you are saying that it can't happen. I highly doubt that is the case, though. Aug 28 '19 at 13:51
  • 2
    I can say "I don't" but it means that it's a repeated action, and you know from past experience that I never forget the bread. (This isn't true; I'm always forgetting the bread). "No, I don't" would be odd though.
    – Strawberry
    Aug 28 '19 at 16:49

So what you seem to be having trouble with are tenses. The present simple tense can be used for things that happen regularly, sometimes, or never, but also for commands.

  1. Don't do that!
  2. I sometimes do it.
  3. I never do so.

1 is a command, whereas 2 and 3 are not. "Don't forget the bread" is a command, just like 1.

Now, when the person listening to the command answered, he used the future simple tense to answer, "I will not", meaning he will not forget the bread on that specific day. "I don't" wouldn't be applicable here as you A can't command yourself, and B it doesn't provide any meaning regarding the timing. Nevertheless, if one would have wanted to use the present simple tense in that case, then the appropriate answer would be, "I never do."

  • This was almost the perfect answer, but missed the explicit explanation of how will/shall is used to form the future simple tense. I've submitted an edit that I think ties that up nicely. Aug 28 '19 at 19:02

The imperative in English uses the same form as the bare infinitive:

Play tennis!
Be good!
Don't watch TV.

You can see in the last example that when forming an imperative with a negative verb, the helping verb "do not" is used. All these imperatives are likely to be talking about future activities. When you say "Play tennis!" the action hasn't happened yet. If the person replies then they are talking about a future action, and so are likely to use one of the future tenses

Play tennis!
Okay, I will play tennis.

Be good!
I'm going to be good until Christmas.

Don't watch TV!
Okay, I won't watch TV.

You see that the indicative reply about the future action uses a future tense. There are several ways of talking about the future in English. In the last example you see why "I won't" is a correct reply to an imperative. It is because you are talking about the future.

Don't watch TV.
No, I won't.

  • Re: "The imperative in English uses the same form as the bare infinitive": This is true of the affirmative imperative, but it's not true of "Don't". (We can't say *"to don't", *"will don't", etc.)
    – ruakh
    Aug 28 '19 at 23:41
  • I'd already noted that in my answer.
    – James K
    Aug 29 '19 at 15:48
  • I've read your entire answer, and literally not one part of it says "By the way, the first sentence in this answer is completely wrong in exactly the case you're asking about, but I said it anyway for no reason." ;-)
    – ruakh
    Aug 29 '19 at 16:45

In English, it's customary to use the simple future, for making promises, especially promises made on the spur of the moment.

Mother: Be careful driving the car
19-year-old son: I will, Mum.

Father: Don't be late back home.
17-year-old daughter: I won't, Dad.

The OP's short dialogue is similar

A: "I have to go shopping this afternoon."

“This afternoon” tells the listener the action will occur at a specific point in the future.

B: "Don't forget the bread."

Shortened equivalent of: [Please] Don't forget to buy the bread when you go to the shops [this afternoon].

A: "No, I won't."

Shortened equivalent of: No, I won't (will not) forget to buy it [the bread].

  • 2
    I don't really agree with your first sentence. Promises are made using the tense appropriate to the time they refer to. In this case, the speaker is promising to do something (not forget the bread) in the future (while shopping), so the future tense is used. In contrast, I promise that I'm sober while writing this comment (promise about the present, made in the present tense) and I promise that I didn't run any red lights on my way into work today (promise about the past, made in the past tense). Aug 28 '19 at 16:36
  • There are no hard and fast rules in English, this answer is about explaining why "won't" was used in this instance.
    – Mari-Lou A
    Aug 28 '19 at 16:47
  • 1
    @DavidRicherby Also in American English. "Mom, I promise you that I didn't drink beer on purpose." Anybody who doubts can search '"I promise that" - will' in a search engine.
    – gormadoc
    Aug 28 '19 at 19:40
  • 1
    I also don't think it's relevant that it's a promise. I think it's normal to use any tense with promise: Mom: "Promise me that you aren't on drugs." Teen: "I promise, I'm not!" or Mom: "Promise me you didn't spray-paint the neighbor's fence." Kid: "I promise, I didn't!"
    – gormadoc
    Aug 28 '19 at 19:49
  • 2
    Listen folks, I said it is customary, not that it's carved in stone. There's also a link in the answer. Check it out. Of course if someone uses reported speech the future is not needed. And a promise doesn't have to have the word "promise" in it, e.g. We'll visit your mother on Sunday and "You promised we would visit mother" uses the past simple tense. "You said we would have visited mother on Sunday" again a different tense, conditional perfect, and the verb "promised" s replaced with "said". "I won't forget" is similar, it's a type of guarantee, a type of promise.
    – Mari-Lou A
    Aug 28 '19 at 20:28

Just to complement the other answers, to understand what's going on here, it's important to understand that the original statement commands you not to do the act of forgetting. So your "I won't" reply can be expanded without changing its meaning to:

"I will not do it."

Which means that you undertake not to forget.

In this particular example it would be easy to conflate the doing of the forgetting with the doing of the shopping, which is probably where the ambiguity arises.

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