2

How can / could you say such a thing?
How can / could you do this to me?
How can / could I work with all this noise going on?
How can / could this be?
How can / could I possibly refuse such a charming invitation?
Surely he can't / couldn't have forgotten about the wedding!
Who can / could be phoning us a this time of night?
What can / could have happened to them?

What's the difference between 'can' and 'could' in the sentences above?

closed as off-topic by Phil, user3169, jimsug, Damkerng T., Esoteric Screen Name Aug 13 '14 at 4:39

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question should include more details than have been provided here. Please edit to add the research you have done in your efforts to answer the question, or provide more context. See: Details, Please." – Phil, jimsug
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2
    Way too many different issues related to modal auxiliaries and modality are involved here. Please just pick one question with one example. Otherwise, perhaps read the many pages that can be found in a decent grammar source that discusses the modal auxiliaries "can" and "could" and the ways they are used. Also, perhaps let us know what you currently think is the difference in the usage of "can" vs "could" for that one example. – F.E. Aug 12 '14 at 23:49
  • What @F.E. said. But for most or all of these examples, in most or all contexts, it's entirely a stylistic choice whether to use present/past can/could (in the "real" world, they're all effectively "current" questions, but that doesn't mean they have to be queried using current/present tense). – FumbleFingers Aug 13 '14 at 3:58
1

Some reading: http://blog.oxforddictionaries.com/2013/07/can-or-could/

Could is the past tense of can.

How can / could you say such a thing?

Almost same meaning. Could puts emphasis on the exact event, can puts emphasis on the ability to say something that bad.

How can / could you do this to me?

Almost same, see above.

How can / could I work with all this noise going on?

Present vs past: "How can I work right now with this noise?" vs "I didn't get my work done; how could I with all this noise?". Could does not exactly fit since "going on" is not a past event.

How can / could this be?

Present / past

How can / could I possibly refuse such a charming invitation?

"How can I ..." sounds a little off. If you didn't have "possibly" then this would start to sound like an actual refusal!

"How could I ..." is better since it places the decision in the past. You are reassuring the person that you will not refuse.

Surely he can't / couldn't have forgotten about the wedding!

No comment.

Who can / could be phoning us at this time of night?

Can does not work since you are talking about a past (even two seconds ago) event. If you change the sentence to "Who can that be?" then it works: you are talking about the current event of finding out.

What can / could have happened to them?

Can does not work, since you are talking about a past event.

1

It looks like the differences above are simply a matter of present or past tense. Note that using "can" vs "could" in some cases might be bad grammar-wise.

How can / could you say such a thing? [focus on general moral ability]

  • "How can you say such a thing?" [present]
  • "How could you say such a thing?" [past tense but sometimes used in present tense]

How can / could you do this to me? [focus on general moral ability]

  • "How can you do this to me?" [present]
  • "How could you do this to me?" [past]

How can / could I work with all this noise going on? [focus on general physical/mental ability especially in a specific situations]

  • "How can I work with all this noise going on?" [present]
  • "How could I work with all the noise going on?" [past]
  • "could" doesn't work without changes.

How can / could I possibly refuse such a charming invitation? [focus on ability to refuse (obligation)]

  • "How can I possibly refuse such a charming invitation?" [present]
  • "How could I possibly refuse such a charming invitation?" [past tense but sometimes used in present tense]

Surely he can't / couldn't have forgotten about the wedding! [focus on general mental ability (to remember)]

  • "Surely he can't have forgotten about the wedding!" [technically present tense but ambiguous]
  • "Surely he couldn't have forgotten about the wedding!" [past tense but likely leading to present]

Who can / could be phoning us a this time of night? [focus on possibility (speculation)]

  • "Who could be phoning us at this time of night?" [past]
  • Since the caller has already placed the call, "can" would not be used.

What can / could have happened to them? [focus on possibilty (speculation)]

  • "What could have happened to them?" [past]
  • Since we would know what happened if we were present, this sentence must be past tense. "can" would not be used.

Not the answer you're looking for? Browse other questions tagged or ask your own question.