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Hi,

'Use of Could

could: Past Possibility or Ability

We use could to talk about what was possible in the past, what we were able or free to do:
  • I could swim when I was 5 years old.
  • My grandmother could speak seven languages.
  • When we arrived home, we could not open the door. (...couldn't open the door.)
  • Could you understand what he was saying?'

I am studying the modals and I want you please to help me with 'could'.
'Could' can be used to describe past possibility and ability, as the quoted part from a grammar site explains. I can't seem to differentiate between the two uses in these examples and other ones.. It seems that I can say that it was possible for me , or I was able to swim ... . Could you help and show me how to tell which one is which?

Regards
N A
Comments  
AnonymousCould you help and show me how to tell which one is which?
You can almost always tell from the context. That is the only way to differentiate. Nothing in the grammar will give you that information. Compare these two:

When the drought came and dried up the lake, we couldn't swim.
My little brother couldn't swim until he was 9 years old.
AlpheccaStarsWhen the drought came and dried up the lake, we couldn't swim.
I was not possible.
AlpheccaStarsMy little brother couldn't swim until he was 9 years old.
He wasn't able.
But what about the examples provided? Which is which? You say from these examples one can't tell.
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AnonymousI could swim when I was 5 years old
Ability. The context is an early age for learning how.
AnonymousMy grandmother could speak seven languages.
Ability. It is not a common skill.
AnonymousWhen we arrived home, we could not open the door. (...couldn't open the door.)
Impossibility. Of course we have the ability to open a door. That's a normal function for a door. But in this case, something failed in the mechanism, so if was not possible.
AnonymousCould you understand what he was saying?
This is ambiguous. If he is speaking in a foreign language, I may not have the ability to understand under any circumstances.

If there is a lot of noise in the room, I may understand the words he says, but it is impossible to hear them.
Anonymous'Could' can be used to describe past possibility and ability, as the quoted part from a grammar site explains. I can't seem to differentiate between the two uses in these examples and other ones.
In practical terms it may be that you don't need to differentiate the cases as long as you know how to use "could". Sometimes "could" in a given sentence can be interpreted in either way, and that's OK.

Pure possibility:

There's a knock on the door, but I can't see who it is. It could be Jack, because I'm expecting him to drop by today.

Using pure logic, I determine that it's possible that the person knocking on the door is Jack. "It" in "It could be" is not a person, so "it" can't be able to be Jack. The abstract "it" is not able to do anything. And even if you say instead, "The person at the door could be Jack", it doesn't mean that whoever is at the door has the ability to make himself be Jack.

Pure ability:

(This one is just about impossible because there must be a possibility before there can be an ability.)

Jack was so strong when he was 18 that he could lift 200 pounds effortlessly.

Jack had the ability to lift 200 pounds. Yes, it was also possible for him to lift 200 pounds. The important thing here is that Jack is a person who could do this if he wanted to. We say that Jack is an "agent" in this scenario. So in the cases of could that focus on ability, there is an agent who was able to do something if he wanted to. Note that this does not apply to the example above. "It could be Jack" can't mean that "it" was able to be (become) Jack if "it" wanted to.
__________

There are even cases with an agent that may seem not to involve ability.

Back in 1870 anyone could buy a buggy whip at the local dry goods store.

anyone (any person) can be an agent, and anyone could do this if he wanted to, but it's debatable whether the focus is on the physical or mental ability to purchase something in this case. Rather, this sentence seems to focus on the possibility that was open to anyone to buy a buggy whip if he needed one.

Nevertheless, nothing of any importance hinges on your ability to distinguish whether such a sentence is more about possibility or more about ability, so I don't recommend worrying over the fine points of this distinction. Emotion: smile

CJ
Thanks CJ; thanks Alphecca.
CalifJimNevertheless, nothing of any importance hinges on your ability to distinguish whether such a sentence is more about possibility or more about ability, so I don't recommend worrying over the fine points of this distinction.
I think it has to do with two factors. First, because of my being a teacher of English, my students asked me to differentiate and point out exactly which of the sentences, in the quoted grammar in my OP, indicated possibility and which indicated ability, as the rule stated, and I was supposed to know. Second, being a learner myself, I felt as curious as my students.
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Anonymousmy students asked me to differentiate and point out exactly which of the sentences, in the quoted grammar in my OP, indicated possibility and which indicated ability
You can probably give them the same explanations we gave you, adding my remark about how unimportant it is to know such fine details. Most native speakers don't know how to classify such cases. Emotion: smile

CJ
Except for people studying grammar as an end in itself, classification/labelling are sometimes useful in explaining how words are used. If the classification labelling becomes difficult , hen it has lost any value it might have, in my opinion.
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