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I have doubt about the correctness of the following rule. If it is correct then is there any reason behind this?

Rule:

Not all abilities are general. Some abilities happen only once (or a certain number of times) in a particular situation. In positive sentences, we can only use "was able to" for the specific event / occasion

Examples:

I was very tired but I could stay awake all night. - This is according to the rule I am investigating syntactically incorrect
I was very tired but I was able to stay awake all night.

If the sentence is negative, we can use could or was able to interchangeably .

I couldn't hear him because of the music.
_I wasn't able to hear him because of the music. ✓

Why are we able to use both "could" and "was able to" in negative sentences, but not in positive ones?

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I'm sorry to say I just don't understand your question. –  jbeldock Apr 25 '13 at 13:43
    
There are some strange things happening with tenses in your "positive sentence" could example, but it is otherwise fine. I was very tired, but could have stayed awake all night. –  dotsamuelswan Apr 25 '13 at 14:07
    
Just out of interest, where did you get this rule from? –  Matt Apr 25 '13 at 14:13
    
I'm afraid you have been misled by yet another BS grammar book. This rule is completely wrong. Throw the book away. –  John Lawler Apr 25 '13 at 15:47
    
There's nothing grammatically or syntactically wrong with it, but I agree that it sounds odd, and I don't think I would say it. But I would if there were more words there: but I found I could stay awake all night, and but I could still stay awake all night, both sound fine to me. I cannot account for why it seems odd to me. –  Colin Fine Jun 7 '13 at 22:45
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5 Answers

Yes, this is a "rule" of English grammar but it has to be understood and applied correctly. The advice to avoid could applies only to those sentences where could refers to an ability to perform a certain particular action on a particular occasion. On this basis, the following sentences are problematic:

? I spent an hour looking for my keys and could find them.

? I could buy the notebook at a good price.

? There was a fire in the building but everybody could escape.

? I played well for a change and could beat him.

In such situations we are more likely to use an alternative such as was able to, managed to, or succeeded in.

Here are explanations and examples from four recent grammars (the first three pedagogic, and the fourth descriptive):

Swan: Practical English Usage

We do not normally use could to say that somebody managed to do something on one occasion:

? After six hours climbing we could get to the top of the mountain.

Murphy: English Grammar In Use

We use could for general ability. But if we are talking about what happened in a particular situation, we use was able to or managed to (not could).

? They didn't want to come at first but we could persuade them.

Carter & McCarthy: Cambridge Grammar of English

When actual achievements are indicated, was/were able to, not could, is preferred in past affirmative cases:

? The thieves escaped but the police could arrest them later that evening.

Huddlestone & Pullum: The Cambridge Grammar of the English Language

An important restriction is that could does not normally appear in affirmative contexts when it is a matter of actualisation of a single situation viewed perfectively:

? I left early and could get a good seat.

The OP's example sentence is somewhat tricky because staying awake is more of a state than a single action. The following sentence is a more clear-cut example of the application of the rule explained above:

? After lying in bed for hours staring at the ceiling I could fall asleep.

Better:

After lying in bed for hours staring at the ceiling I managed to fall asleep.

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I'm not a native speaker, but when I see OP's sentence "I was very tired but I could stay awake all night.", the 'could' makes me think of possibility, not ability. Basically it's not that 'I had the ability to stay awake' (meaning something controlled by me), but that 'I had the possibility to do so' (meaning something not controlled by me, but by others - people or situations). –  Sara Costa Jun 8 '13 at 13:50
    
@Sara. Yes, you can use could in affirmative sentences when it has the meaning of permission. For example: The child was very tired but could (was allowed by her parents) stay awake all night. This still sounds a little strange to my native-speaker ears. More acceptable is: She was very spoiled and could do what she wanted. –  Shoe Jun 8 '13 at 19:42
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"Could" and "was able to" can be used in both negative and positive sentences alike. This sentence, which you assume is incorrect, is actually perfectly valid:

I was very tired but I could stay awake all night.

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Thanks I had afraid too after reading this rule –  Satyam Apr 26 '13 at 3:55
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The premise of your question is wrong. You can use "could" and "was able to" in positive sentences:

I was very tired so I wasn't able to stay awake all night.

After a coffee that strong, I could stay awake all night!

Despite how tired I was, I could stay up all night.

I could hear him perfectly well until the noise outside started.

I was able to hear him because of my hearing aid.

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I'll put that down as idiopathic. The positive/negative "rule" is not correct. Yet there's a complication with tenses.

Not correct:

I was very tired but I could stay awake all night.

Correct:

I am very tired but I could stay awake all night.

Go figure!

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What about will?

  • I am tried but I will stay up all night.
  • I was tired but I was able to stay up all night.
  • After a cup if coffee, I will be able to stay up all night.
  • I am tired but I can stay up all night.

I believe these selections display more will power than ability and brings assurance to the person who is receiving the message. The fact of "could" seems like a possibility rather than an assurance. To me assurance always sound more positive than a risk possibility. I sort of understand where this is going especially when working in customer service and hospitality. Could doesn't give assurance, able signals ability which is quite more positive. You know what you are capable of and does allow the person whom you are communicating to understand this more firmly. I usually use will to make someone feel the will power or can do attitude. Although, if will is used incorrectly it can sound limiting.

Here is an example. The situation is that computer systems are down. Guest: "I booked my hotel room online for $250 to two nights. However, it shows on my receipt I am suppose to check out in the morning."

— I will definitely look into that as soon as our systems are back up.
— I am able to look into that for you as soon as our systems are back up.
— I could look into that for you once our systems our back up.

Although all three sentence does say similar things, each once creates a different response.

  • Will: customer is assured you will do it and most likely will say thank you and move on.
  • Able: let the customer know nothing can be done right now but will possible been done as soon as I am able but no assurance.

This same affects place with "Could" as well. However, saying could sound like I could but will I. Could sounds like personal choice instead of duty.So I usually avoid could. Because could means come back, check with me later to see if it up when I maybe able to do it.

Able speaks of ability. When the customer return and the system is up I am able to do it now. I wasn't able too earlier.

Will signals that I will keep that mind. Most likely when you return I will already have the resolution.

I feel that able is more positive because you limited by constraints that are not of your own will.

  • I will give you a ride later to the store.
    I will give you $10 as soon as I have cash.
  • I am able to give you a ride later to the store.
    I am able to give you $10 as soon as I have cash.
  • I could give you a ride later to the store.
    I could give you the $10 as soon as I have cash.

If someone said those to you, who will have a more positive response with? It's all about perception of course because people sometimes understand them interchangeably and others will misread what you trying to say. I have worked with many people and I have found that will takes you further with people because it removes doubt of the unknown. This is just my point of view, of course.

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The question is about the past tense. Will in your examples is not past tense, so your answer doesn't really apply here. –  Matt Эллен Dec 16 '13 at 9:23
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