About future probability:
It couldn't rain tomorrow.
It can't rain tomorrow.
It couldn't possibly rain tomorrow.
It can't possibly rain tomorrow.
About present probability:
It couldn't be true.
It can't be true.
May I ask what is the difference among those sentences? Are the meanings of the ones using "could" are less definite than the ones using "can"?
Please give me your opinion and I'll appreciate it.
Simplest explanation on usage (I hope it's simple XD):
Could or Couldn't: Talking about the possibility or lack of a possibility. "It couldn't rain today" = "There's no possibility of it raining."
Can or Can't: Talking about the ability or lack of ability: "It can't rain today" = "It is not able to rain today."
For instance, you can say that it "couldn't" rain, but there still may have been rain clouds over head. Conditions just happened to not allow for it to actually rain. If you say it "can't" rain, there is virtually no possibility that it will--there are probably no clouds in the sky that will actually drop any rain.
More technical explanation:
"Can" is a regular verb. "Could" is the conditional form. From a dictionary stand-point, they might carry the same basic semantic meaning.
We often say "can't" or "couldn't" to deny the possibility of something, with only a subtle difference between the two. "It can't be true" is the speaker not wanting to believe a real fact. "It couldn't be true" is the speaker not wanting to believe that something MIGHT be true, i.e. it may or may not be an actual fact.
Hope that all makes sense ^_^;;;
Nooooooooo, no, no....this is not a issue of tense.
"can" is actual ability
"could" is possibility (a conditional)
"I can lift 100 pounds." This speaker is assured of his ability to lift that much weight.
"I could lift 100 pounds." This speaker is saying he is likely to be able to but there may be some conditions that go along with it.
It's probable. Such as "I could life 100 pounds, but I might break my back." Or "I could lift 100 pounds, if you paid me enough."