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We use COULD to talk about things we are able to do generally. We don't normally use COULD for something that happens on a particular occasion in the past.We use was able to or managed to do"

Source: Oxford Practice Grammar

Now, in the same book, in the PRACTICE section, there was an example below:

Last year Robert COULD beat his brother at chess. But he can't beat him now.

I have a question: Why does the writer use 'COULD' instead of 'was able to' or 'managed to do something', because the example above is not general situation it is a particular occasion.

My analysis:
I feel that although that situation happened last year, it wasn't happening during a whole year. For example, every week, every month or every day. Playing chess and beating her brother just happened on a specific day of the year or a specific month of the year, not the entire year. That is why I think it is not a general situation.

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    'Last year' is far too lengthy to be considered to refer to 'a particular occasion. 'Last year Robert could beat his brother at chess.' means 'Whenever they played chess last year, Robert generally won.' – Edwin Ashworth Apr 24 '17 at 8:30
  • Last year Robert could beat his brother very strongly implies that Robert did beat his brother last year (probably repeatedly, as per @Edwin above). Last year Robert was able to beat his brother could be used with the same implication, but it could also feasibly be used with the sense of Last year Robert would have been able to beat his brother or Last year Robert could have beaten his brother (in principle he had that ability, but he never actually played and beat his brother). – FumbleFingers Reinstate Monica Apr 24 at 16:05
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Last year Robert COULD beat his brother at chess. But he can't beat him now.

He could beat his brother any time they played chess last year, not just on one particular occasion. Since this is something he could do "generally" over an extended period of time, you can use either "could" or "was able to".

If you're talking about one particular occasion, then there's no question of whether he "could" beat his brother --- the event has already happened and the outcome has been decided. Then, we use the phrase "was able to" to emphasize it was challenging to achieve this result. The quote you included is telling you you can't replace "was able to" with "could" in a sentence like this:

Todd was able to beat his brother at chess when they played on Christmas eve last year.

This means that Todd won this particular game, but doesn't give any information about other games they played last year.

If you wrote

Todd could beat his brother at chess when they played on Christmas eve last year.

it implies they played multiple times on that day.

You wrote,

I feel that although that situation happened last year, it wasn't happening during a whole year. For example, every week, every month or every day.Playing chess and beating her brother just happened in a specific day of the year or a specific month of the year, not the whole year. That is why I think it is not a general situation.

This isn't correct analysis. By using "could", the writer implied this was a habitual or general situation throughout the year (or at least for a large portion of the year). It doesn't just refer to one game.

If discussing a specific game, they would have just told you he did win or he didn't. You wouldn't have to talk about it as a possibility.

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    Your excellent and detailed answer covers all the angles and actually helps the OP to go through the process of answering the question. – English Student Apr 24 '17 at 6:24
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    I think this is over complicated. Regardless of what the Oxford Practice Grammar says people do (in the real world) normally use 'could' to mean 'was able'. In my experience, the former is more common. – JimmyJames Apr 24 '17 at 16:54
  • @JimmyJames "Could" does indeed mean "was able," at least in this context, but if I say that last year Robert was able to beat his brother at chess, I may be referring to something that Robert did on just one occasion and that he was never able to repeat. – David K Apr 24 '17 at 18:17
  • @Doc, no, because "when they played" tells you they actually played. To express the meaning you suggest, you'd say "Todd could have beaten his brother, had they played on Christmas Eve." – The Photon Apr 24 '17 at 22:05
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Although not strictly defined in formal grammatical rules, 'could' basically functions as a past tense for the modal 'can'. (See the linked article as to why it isn't completely a past tense part of speech)

I was able to climb a tree when I was younger, but I'm not able to anymore.

I could climb a tree when I was younger, but I can't anymore.

The sentence means that, during all of last year, Robert was capable of beating his brother at chess. It's not really behaving as a conditional modal, like 'would', 'could' and 'should'.

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    Right. saying 'could' is incorrect here is extremely pedantic especially in informal communication. – JimmyJames Apr 24 '17 at 16:50
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    +1. I've always considered using could as past tense of can to be just fine, and a lot of people accuse me of being pedantic. – Monty Harder Apr 24 '17 at 18:26
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    These examples both discuss an extended period of time, "when I was younger", not a specific occassion (like "that time I needed to get my kite out of the tree"). So this doesn't contradict anything that OP's reference or the other answers have said. – The Photon Apr 24 '17 at 22:08
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I think the real issue here is that in common usage, "could" tends to mean or imply one of two things:

  1. A potential event in the present, similar to "can" ("I could stop smoking, but I don't want to.")
  2. A consistent event only in the past, the inverse of "can" ("I could hit three homers a game in my prime.")

The second part of these implications is what's important here. The "present" version is a slangy replacement for "can" that suggests that the subject is still able to perform the actions stated, while the "past" version explicitly informs that the subject cannot still perform the action stated.

The differences are (probably poorly) highlighted here, in the same order:

  1. "I could bowl a perfect game if I had my lucky ball."
  2. "I could bowl a perfect game, but then I broke my finger."

The former is conditionally stating an event is possible in the present, while the latter is unconditionally stating an event is no longer possible in the present.

They're contradictory definitions without context, but if you add a time frame, it makes it clear.

  1. "Could" + present = "can, uncertainly"
  2. "Could" + past = "did, certainly"
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Your mistake in reading the sentence originally was assuming that it was used IMPROPERLY, rather than accepting that it was used PROPERLY to convey information not explicitly stated.

Since COULD is ONLY properly used for general situations, not specific instances, the sentence structure and use of COULD tells you several things.

  • The brothers played chess at least once, most likely frequently, during the year referenced
  • One brother was able to win the games more habitually than the other
  • The general situation has altered since the time frame indicated

This is an important aspect of what is often called "reading between the lines." It allows the writer or speaker to imply or express concepts and circumstances never actually stated by utilizing grammar in precise ways.

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    I do not agree with one of your conclusions: One brother was able to win the games more habitually than the other. Could implies possibility - it does not imply a particular frequency. For example, last year Lee Se-dol could beat DeepMind in Go. He won one game and lost four. However, the statement would be equally correct if he only lost one game and won four. – jkhan Apr 24 '17 at 16:55
  • @jkhan I think this is an ambiguity in the meaning of can. We might say can when we mean "is always (or usually) able to," but in your example it means "is sometimes able to." – David K Apr 24 '17 at 18:25
  • @DavidK It's probably also worth pointing out that COULD may be used even in cases where a particular outcome may not have ever occurred, but was possible. For example, I have never actually won a game of chess against my brother, but they were always close. I could have beaten him last summer. Compare to: I could never imagine beating Gary Kasparov in a game of chess. Context will dictate the meaning - from the context given it is not safe to assume one brother won more often than the other. – jkhan Apr 24 '17 at 19:58
  • @jkhan True, "had the ability to" does not necessarily imply that it happened when you tried it. – David K Apr 24 '17 at 20:41
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Both sentences are possible, but the meaning is not the same.

Last year Robert managed to beat his brother at chess.

This sentence implies that the brothers played once, and Robert won. This would be perfectly valid in a context where e.g. they are known to meet once a year.

Last year Robert could beat his brother at chess.

means that the brothers played multiple times through the year, and Robert won several games.

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