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We ___ need more sugar already - I only bought some last week.

The key suggests can't should be used in question NO.4, but it doesn't make sense to me.

I wonder if the key goes wrong? I may just say don't.

And what does it mean if can't is used here?

Anyone can help?

Complete the following sentences using an appropriate modal verb.

  1. All children under 12 ______ be accompanied by an adult.
  2. There's a free 30-day trial period, so you ______ pay anything for the first month.
  3. I missed the last train and ______ get a taxi home.
  4. We ______ need more sugar already — I only bought some last week.
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  • 5
    Please re-type the question into text. Images are not searchable, indexable or readable without eyes.
    – James K
    May 30 at 12:11
  • Modal verbs: (1) must (2) needn't, (3) had to (4) can't for anyone who's stumped on (3) :)
    – Mari-Lou A
    May 31 at 16:58
  • Is I only bought some last week British usage? It sounds strange to my western American ear. I think I just bought some last week would sound better?
    – Ron Jensen
    May 31 at 18:06
  • To this British ear, ‘only’ sounds fine — but it would sound even better next to ‘last week’, e.g. “I bought some only last week.”
    – gidds
    May 31 at 18:59
11

We can use "can't" to express disbelief about something, or surprise that something is doesn't match your assumptions.

That can't be the train. It isn't due for another 15 minutes.

It can't be a ghost. They aren't real.

You can't be hungry already. = "I assumed that you are not hungry (because you ate recently) but now you want to eat again. I am surprised.

It is often paired with "already", and is used in situation where a child is hungry again, or something has already been used.

So this is idiomatic and means "I'd assumed that we would still have sugar (because I bought some recently), but we have run out and I am surprised at this."

"Don't" isn't a modal verb, but it could be used in that position.

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    To my ear (BrE), “don’t” sounds wrong here: it should be paired with “yet” rather than “already” May 31 at 12:23
  • “We can't need more sugar already” doesn't seem really idiomatic either. “Can't be” is common, yes, but “can't need”? Perhaps “can't be needing”, but that's clunky. “We can't be out of sugar yet” gets to the point best. (Short of “no way ain't no sugar left no more”...) May 31 at 19:59
5

"Can" is used to say something is possible.

"Can't" is used to say something isn't possible. Although in practice it's often used to express disbelief and it often means something similar to "shouldn't".

"Do" is used to make a statement (roughly speaking). But it is only really included to specifically emphasise that something is in fact true (or when it's negated). "We need more sugar" and "We do need more sugar" means the same thing, except for the emphasis.

"Don't" is used to negate a statement, e.g. negating "We need more sugar" gives us "We don't need more sugar".

We ___ need more sugar already - I only bought some last week.

In this sentence you could probably use either "can't" or "don't", but the rest of the sentence implies disbelief at the fact that more sugar is already needed, so "can't" is probably more appropriate.

For "don't", I'd probably change it into a more neutral statement:

We don't need more sugar yet. I bought some last week.

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    Just to add for clarity, when you suggest the alternate sentence for "don't", it has a different meaning than the one in the question. It's not a way to rephrase the sentence with different words. May 31 at 2:08
  • 3
    Chris's comment is correct. However, "shouldn't" could also be used and would have a more similar meaning. "Can't" in this sense is an expression of incredulity. "Shouldn't" is an expression of doubt. "Don't" would be an assertion. That said, looking only at the provided context, I could easily argue that "don't" works.
    – Matthew
    May 31 at 2:53
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    @Matthew: I don't think that don't would work in this sentence, because then we'd expect yet rather than already. (This is a bit subtle, admittedly. I wouldn't expect someone learning English to pick up on it.)
    – ruakh
    May 31 at 7:48
3

In this context, "can't" It's part of the same grammar as "might/may/could/must" to express how likely something is; "can't" means it's logically impossible.

In a sudoku puzzle, you might say, "This square might be a 3 or a 5, but it can't be a 7 because there's already a 7 in this row."

Or in answering a multiple choice question, "The correct answer can't be A or B because they're both past tense but the sentence is about the future."

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The difference between "don't" and "can't" here:

  • We don't need more sugar: I know for a fact that we have sufficient sugar. (I have just looked in the cupboard).
  • We can't need more sugar: I think it's likely that we have sufficient sugar. (It's only been a week, so unless someone has been baking cakes without me noticing, then logically we still have enough.)

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