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Excerpt from BBC 6 Minute Grammar:

Catherine
If the weather gets worse, our plane could take off late. We might not take off at all!

Neil
Now that’s all fine, but if you say: We couldn’t take off – you’re talking about an impossible situation in the past, not an uncertain situation in the future. So you can’t use couldn’t in our airport example.

So they say that the following sentence is incorrect:

If the weather gets worse, our plane could not take off.

But is it true? Perhaps the weather is already bad and if it gets worse I am certain that it would be impossible to take off (i.e. we couldn't take off).

In the same programme they also say:

CORRECT: The traffic is getting worse, so I may not be home on time.
WRONG: The traffic is getting worse, so I could not be home on time.

But perhaps I can see the situation and I am certain that it would be impossible to get home on time (i.e. I could not be home on time).

And the last one from the same unit of the course:

We don’t use couldn’t in the same way as might not/mightn’t/may not. ‘Couldn’t’ means that something is impossible.

WRONG: I think there couldn’t be any trains today, so I’m going by bus.
CORRECT: I think there mightn’t be any trains today, so I’m going by bus.

Well, probably I can see on the timetable that the last train has already left and there won't be any trains today. So here it's impossible to be any trains today (i.e. there couldn’t be any trains today).


So my general question is: what is wrong with using couldn't for future impossibility in these examples?

  • Well, as for the last example now I am thinking that the key part is I think, because it may suggest that actually I'm not certain about the impossibility. But it doesn't explain the first two examples anyway. – Karolini Sep 19 '17 at 10:01
  • "What is wrong with using couldn't for future impossibility" - other than the fact that we just don't use couldn't that way? We use won't or can't. Maybe I'm misunderstanding your question? – stangdon Sep 19 '17 at 11:55
  • @stangdon Well, but this sentence uses couldn't for future impossibility and I know that this sentence is correct: I couldn't live in a big city. I'd hate it. – Karolini Sep 19 '17 at 12:01
  • Actually, "I couldn't live in a big city" is technically a statement about the present, not a future: I couldn't do it right now, but maybe in the future I could. – stangdon Sep 19 '17 at 12:15
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    If I had to make that kind of statement explicitly about the future, I would say, "I won't be able to live in a big city in the future." Loosely, I couldn't = past, or present hypotheticals; I can't = present; I wouldn't = hypotheticals. I'd only use wouldn't in a case like "I wouldn't be able to live there if the weather got any worse." – stangdon Sep 19 '17 at 15:24
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"Could" is a multi-purpose word. Sometimes it indicates potential:

He's a swimming prodigy. He could be the next Michael Phelps

Sometimes opportunity:

We could have dinner at that new restaurant which just opened over on 5th street.

And sometimes simple past ability:

There was too much traffic, so I could not get to the post office today.

In your examples you're mixing together potential and ability. If you want to focus on a possible future action, "could" is fine:

Could the plane take off before the bad weather rolls in?

But not if we're talking about the ability to perform some action:

The plane can't land on that runway. It could have landed on the one a hundred miles back, but now there's not enough fuel to go back.

Of course, sometimes there's almost no distinction between "potential" and "ability". Your train schedule is a good example. It's fine for me to ask:

Could there be another train?

because this refers to the possibility of more trains. But how about if I ask:

Could I take the next train?

Am I asking if it is possible for me to take the next train? Or am I asking if I am able to take the next train? Is there really a difference between these? If there are no more trains, you could answer either way:

No you can't (you're not able). The last train left ten minutes ago.

No, you couldn't (it's not possible). The last train left ten minutes ago.

To be clear, when specifically talking about ability, "could" only refers to past ability:

He can play the trombone.
He could play the trombone when he was in grade school, but now he can't.

As with anything else in English, there may be exceptions.

  • Thank you so much for you contribution. Let's get back a bit to one of my previous examples in my main post: "The traffic is getting worse, so (now I am certain) I could not be home on time (unless it got better)". Here unless-clause is an implicit part of the sentence which makes it absolutely valid second conditional sentence for hypothetical future (or also present) impossibility. But I may not understand something. – Karolini Sep 21 '17 at 16:00
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    @Karolini I'm trying to explain what the use of "could" implies, but not necessarily how people actually use it. Your example is fine, but I would be more likely to express it as I am unable, "I can't get home on time, unless the traffic clears up." – Andrew Sep 21 '17 at 17:37
  • @Karolini I'm American, though, and the BBC is British English. They may be more likely to use "could" in that situation, "I couldn't make home in time for tea unless the traffic improves." – Andrew Sep 21 '17 at 17:37

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