Can't and couldn't for impossibility

taraa

Senior Member
Persian
Hi,
Can 'couldn't' be used in place of "can't" in both sentences below?
1- She can't be very nice if no one likes her.
2- This answer can't be right. It most be wrong.
 
  • taraa

    Senior Member
    Persian
    Yes I know, but 'might' is past and we can use them interchangeably in a sentence like:
    "It may/might be wrong"
    why shouldn't we use 'could' and 'can' interchangeably in that sentence?
     

    dona83

    Senior Member
    Italian
    You should change the sentences as follows:
    1 - She couldn't be very nice if no one liked her. ( you are talking about a situation in the past )
    2- That answer couldn't be right. It must have been wrong.

    By the way 'might' can be used in the present tense to indicate possibility i.e. She might be busy, she is not answering her phone.
     

    taraa

    Senior Member
    Persian
    You should change the sentences as follows:
    1 - She couldn't be very nice if no one liked her. ( you are talking about a situation in the past )
    2- That answer couldn't be right. It must have been wrong.

    By the way 'might' can be used in the present tense to indicate possibility i.e. She might be busy, she is not answering her phone.
    'could' can be used in the present tense to indicate possibility too.
     

    dona83

    Senior Member
    Italian
    I believe can and could can be used in the present to talk about possibility.
    Can is used for a generic possibility or fact, could for a less definite possibility or options.
    i.e. When you go to a friend's party you can meet lots of people. VS You could be right, it looks like it's going to rain.

    However, to express impossibility in the present you can only use can't. Couldn't has to be used to refer to the past.
    Check this:
    can or could
     

    taraa

    Senior Member
    Persian
    I believe can and could can be used in the present to talk about possibility.
    Can is used for a generic possibility or fact, could for a less definite possibility or options.
    i.e. When you go to a friend's party you can meet lots of people. VS You could be right, it looks like it's going to rain.

    However, to express impossibility in the present you can only use can't. Couldn't has to be used to refer to the past.
    Check this:
    can or could
    Thank you
     

    taraa

    Senior Member
    Persian
    What is the difference?
    1- I could say that I didn't want to work, but I would be fired.
    2- I can say that I don't want to work...
     

    dona83

    Senior Member
    Italian
    1- In this case Could it's more an option than a possibility. The correct sentence is either ' I could have said that I didn't want to work, but I would have been fired.' (past situation) or I could say that I don't want to work, but I would be fired. (present)

    2- it doesn't work if you say ' I can say that I don't want to work, but I would be fired ' as you are not talking about a generic truth. It could work if you said 'From my previous experience I can say that I don't really want to work on Sundays'
     

    taraa

    Senior Member
    Persian
    Thank you.

    What is the difference? I don't know 'could' is used for being politeness or for possibility.

    -I Can say "I am Tara in Spanish"
    -I could say "I am Tara in Spanish"
     
    Last edited:

    Scrawny goat

    Senior Member
    English - Ireland
    Hi Scrawny,
    what do you mean? I can say ' I couldn't find my car keys yesterday.'
    I am disagreeing with the comment I have quoted, and displaying a use of ‘couldn’t’ which is in the present tense.
     
    Last edited:

    SevenDays

    Senior Member
    Spanish
    Hi,
    Can 'couldn't' be used in place of "can't" in both sentences below?
    1- She can't be very nice if no one likes her.
    2- This answer can't be right. It most be wrong.
    Couldn't and can't both can be used to express "impossibility" in the present time. The only difference is semantic: for example, can't suggests more certainty (That can't be Mike at the door; I just spoke to him on the phone) than couldn't (That couldn't be Mike at the door; I just spoke to him on the phone).

    Now, we might be less likely to use "couldn't" in your examples because there's a "certainty" already implied in "if no one likes her" and "It must be wrong." Accordingly, the default choice is "can't," precisely for the "certainty" that it conveys.

    It follows that this is not really a question of "tense." In modern English, modal verbs don't express "tense" (though, etymologically, for example, could was once upon a time the past of can). More importantly, in modern English, modal verbs have their own syntax and uses. While could and couldn't can be used with reference to past and present times, can only makes refer to the present time.
     
    Last edited:

    se16teddy

    Senior Member
    English - England
    Yes, she couldn't be nice if nobody likes her is grammatical, I think, but the logic is a bit convoluted.

    It means If nobody likes her we can tentatively conclude that she is not nice. It is hard to say why anyone would prefer that to the logically much simpler Yes, she can't be nice if nobody likes her, which means that If nobody likes her we can conclude that she is not nice.
     

    Scrawny goat

    Senior Member
    English - Ireland
    There is another expression people use for this assumption that the herd must be correct: ‘She can hardly be very nice if no one likes her.’
     

    SevenDays

    Senior Member
    Spanish
    What's peculiar about "can" is that it only appears in a negative environment (in contexts where "deduction" is involved). That's what happens in She can hardly be very nice if no one likes her, where "can" is licensed by the negative sense of "hardly." If there's no "negative environment," "can" takes on a different meaning, so that in She can be very nice if everyone likes her, "can" now means "willingness" (and not "deduction").
     

    taraa

    Senior Member
    Persian
    Couldn't and can't both can be used to express "impossibility" in the present time. The only difference is semantic: for example, can't suggests more certainty (That can't be Mike at the door; I just spoke to him on the phone) than couldn't (That couldn't be Mike at the door; I just spoke to him on the phone).
    .
    Can you please explain the use of "couldn't" in the present? How is that different from this "we use can't, not couldn't, to say that something is theoretically or actually impossible."?
     

    SevenDays

    Senior Member
    Spanish
    Can you please explain the use of "couldn't" in the present? How is that different from this "we use can't, not couldn't, to say that something is theoretically or actually impossible."?
    Let's put it this way.

    Both can and could refer to "possibility/probability." So, whenever the focus is on "possibility," both modal verbs are viable in present time:

    She can't/couldn't be very nice if no one likes her

    What's the difference? Modal verbs aren't restricted to one meaning; an additional meaning of can is that it signals greater "certainty" than could.

    When your grammar books says

    we use can't, not couldn't, to say that something is theoretically or actually impossible

    what it's really saying is that "possibility" isn't viable (possibility equals "zero," so to speak). When that happens, we naturally use "can." Under what circumstances is there "zero" possibility? Consider this: a teenage girl is getting ready to go to a party; his father comes in and says:

    You can't go to the party because you have to study for your English exam.

    Here, "couldn't" is automatically ruled out; we don't use it. As far as the father is concerned, the possibility of going out is "zero" because he has just given an order: the girl is not allowed to go to the party. (Of course, the girl could disobey her father and still go out, but that's another story.)

    Often, "possibility" isn't the only factor involved in actual uses of language. In the above example, an "order" is also a factor. Now, consider this; a nurse says:

    The doctor can't see you; he is examining a patient.

    In this example, "couldn't" is also ruled out. Perhaps the doctor has told his staff that he only sees one patient at a time; or perhaps the nurse has decided on her own that the doctor doesn't see anyone else while he is examining a patient. Whatever the case, as far as the nurse is concerned, the possibility of seeing the doctor while he is with another patient is "zero."

    Now, suppose the nurse says

    The doctor couldn't see you even if he wanted to; he is examining a patient.

    That's couldn't used in present time. What happens is that "even if he wanted to" opens the door to "possibility" (and thus the use of "could"), but even considering this small possibility, the doctor still won't see you. Notice that you can also use "can:"

    The doctor can't see you even if he wanted to; he is examining a patient.

    As I said earlier, "can't" signals greater certainty than "could."
     

    taraa

    Senior Member
    Persian
    I understand, thanks a lot SevenDays for the good explanation
    Is "couldn't" for impossibility in the present time always conditional? Or it's just less certain than "can't"?
    "That couldn't be John at the door"--> how is this conditional, please?
     

    SevenDays

    Senior Member
    Spanish
    I understand, thanks a lot SevenDays for the good explanation
    Is "couldn't" for impossibility in the present time always conditional? Or it's just less certain than "can't"?
    "That couldn't be John at the door"--> how is this conditional, please?
    That's not a conditional "couldn't;" it simply expresses less certainty.
     

    se16teddy

    Senior Member
    English - England
    You are still using the word "impossibility" to refer to two different kinds of impossibility, which English grammar treats as fundamentally different.
    1 #24 talks about the use of "can" and "could" to refer to permission (for example permission to go to the party) and surmounting obstacles (such as the barrier presented by being with another patient)
    2 "That couldn't be John" is an example of the use of "could" to talk about what we can deduce from available evidence (such as what we know about where John is)

    Almost nothing that can be said about case 1 can also be said about case 2.
     
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