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Hi,

I have trouble with the modal verbs 'can' and 'could' in some of their functional capacities. I know that 'can' be used to denote possibility and ability like below:

He can drive a truck. -- denote ability
He can grow up to be a CEO. -- denotes possibility

'Could' can also cover the possibility aspect too, I think.

It could flood the city. -- denotes possibility

How can I distinguish between those two 'possibility' uses more clearly?? Sometimes it is difficult to choose 'can' or 'could' in the 'possibilty' departement (if this is the right word to use in this context).

Also, how can you see which one to use in this example for the 'can' and 'could' 'possibility' department?

Can you pass me the napkin?
Could you pass me the napkin?

I feel those two are the same except the 'could' one is more polite -- that to me is the main and possibly the only difference.
But the confusing part is whether 'can' is indicate one's ability or possibility as 'can' can cover both grounds.

What is it? Possibility or ability in action for the modal 'can'?

Can you pass me the napkin?
Can you lift this box?

Clearly, to me, no. 2 denotes ability, ability to lift the box, but the first one is not clear as to whether it is denoting ability or possibility? Maybe he can pass the napkin to another person due to his physical deficiency (?).
Comments  
It's all context. There are no fast rules in English for stuff like this.

Buy Swan, Practical English Usage. You need it.
Read some of the threads here:
http://www.EnglishForward.com/search/can+could+.htm
Sometimes it is difficult to choose 'can' or 'could' in the 'possibilty' departement (if this is the right word to use in this context).

Also, how can you see which one to use in this example for the 'can' and 'could' 'possibility' department? Either one. But both are 'ability' concepts.

Can you pass me the napkin? = Are you able to pass me ...
Could you pass me the napkin? = Would you be able to pass me ...

I feel those two are the same except the 'could' one is more polite -- that to me is the main and possibly the only difference. Correct. The difference is in the politeness.
But the confusing part is whether 'can' is indicate one's ability or possibility as 'can' can cover both grounds.

What is it? Possibility or ability in action for the modal 'can'? Ability.

Can you pass me the napkin?
Can you lift this box?

Clearly, to me, no. 2 denotes ability, ability to lift the box, but the first one is not clear as to whether it is denoting ability or possibility? Maybe he can pass the napkin to another person due to his physical deficiency (?). The point is that he can certainly pass the napkin. His being able to pass the napkin is necessary for the actual passing of the napkin. We question someone's ability to do something as a polite way of commanding them to do it -- not because we want to know if they have the ability to do it, because we already know they have the ability to do it. When you ask someone if they can pass you the napkin, you are not expecting a 'yes' or 'no' answer! You're expecting them to pass the napkin!
Can you pass me the napkin? = Are you able to pass me the napkin? = Pass me the napkin!
Can you lift this box? is not in the context of a typical social exchange, so this one is more likely to be an inquiry about someone's strength -- though it may also be taken as a command.
Can you lift this box? = Are you able to lift this box? can mean Lift this box! or Do you have the ability (strength) to lift this box? depending whether the speaker needs a favor or is assessing his competition, for example, in a contest of strength.
CJ
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Thank you, CalifJim and Marius.

Kudos to you, CalifJim, for possessing such a great degree of knowledge in a confusing area like this and for your ability to give what I think is a very lucid/clear explanation. If I still have doubt, I will post again.
Hi,

Since I wrote a complement of a sort, which still stands firmly, I came to have one more question.

In the Englishpage.com website in its Modal Verb Tutorial section, of all the list functions of the modal 'could', I could not find anything that said 'ability' although it had the 'past ability' category. Why is that?

Pasting a part of my inquiring post and your response for it:

Also, how can you see which one to use in this example for the 'can' and 'could' 'possibility' department? Either one. But both are 'ability' concepts.

Can you pass me the napkin? = Are you able to pass me ...

Could you pass me the napkin? = Would you be able to pass me ...

I feel those two are the same except the 'could' one is more polite -- that to me is the main and possibly the only difference. Correct. The difference is in the politeness.
But the confusing part is whether 'can' is indicate one's ability or possibility as 'can' can cover both grounds.

What is it? Possibility or ability in action for the modal 'can'? Ability.

Can you pass me the napkin?
Can you lift this box?

Also, can you shed some light on the use of the modals for the possibility functional part' for the modals 'could' and 'can'?
How can I make correct choices here when clear distinctions seem to hard to make?

Possibility department of the two modals:

These frequent, continuing absences could affect his ability to bring home bacon.

These frequent, continuing absences can affect this ability to bring home bacon.
These frequent, continuing absences could/might affect his ability to bring home bacon. [Use this one when you want more DOUBT on the ability/potentiality to affect his ability to ..., when you see that as a hypothetical situation.

These frequent, continuing absences can/may affect this ability to bring home bacon.


May/might is mainly related to possibility/potentiality of affecting his ability to bring home bacon
Can/could is mainly related to the capability/ability/power of affecting his ability to bring home bacon
but the usage is mixed lately. Esp the AmE uses a lot more can/could for possibility, where may/might is used by BrE.
You may want to read CJ's post here:
can and could
Also, read:

http://www.merriam-webster.com/dictionary/can
http://www.merriam-webster.com/dictionary/could
http://www.merriam-webster.com/dictionary/may
http://www.merriam-webster.com/dictionary/might
Site Hint: Check out our list of pronunciation videos.
>In the Englishpage.com website in its Modal Verb Tutorial section, of all the list functions of the modal 'could', I could not find anything that said 'ability' although it had the 'past ability' category. Why is that?
Always post the link, so we don't have to look for it:
http://www.englishpage.com/modals/could.html
You have:
I could run ten miles in my twenties.

I could speak Chinese when I was a kid.

listed in the past ability there, but you can easily turn them into present/future ability:
I could still run ten miles anytime these days.
I could speak Chinese again, if you bring two or three Chinese to my table, to spark my recalling of the language.

Could is a modal verb, you can move it from past time to future time with no difficulty.

Instead of reading a lot of grammar, better start reading fiction. All these uses are there, you just need to exercise imagination. English doesn't have a very formal grammar, so it's no use to spend a lot of time on the theory of it. Better focus on the applications and their exceptions, which are infinite.