Anonymous“I think it is wrong to say
we can use the modal "could" to denote ability”
Why do you think it is wrong? Naturally, you need something in the context to cue the reader to the past, but other than that,
could certainly is used to denote ability in the past.
When Jack was ten years younger, he could lift 200 pounds; not anymore. [He used to be able to lift 200 pounds.]
Ten years ago I could not speak English; now I can. [I was not able to speak ...; now I am able to ...]
Anonymous “we can use "could" to denote past ability like as in "He endured so he could play soccer with us."”
That's not really past ability. It doesn't mean
... so (that) he used to be able to play soccer with us. It means
... so (that) he would be able to play soccer with us.Anonymous “When we use the modal "could," the ability we can use it denote is in the past and not in the present although the modal "can" be used for that purpose?”
It's not clear what you're saying
can can be used for -- the past or the present. But maybe that's not relevant. Here are some basics that may clear up some of your confusions:
can is used for the present and for the future of the present.
I can walk for hours without tiring out. [I am able to walk ...]
I can meet him tomorrow. [I will be able to meet him ...]
I'm saving my money so I can buy a new coat. [so that I will be able to buy a new coat]
If I save my money, I can buy a new coat. [I will be able to buy a new coat]
_________
could is used for the past and for the future of the past.
[When I was young / Last year] I could walk for hours without tiring out. [ I used to be able to walk ...]
(I said that) I could meet him the next day. [ (I said that) I would be able to meet him ...]
I was saving my money so I could buy a new coat. [so that I would be able to buy a new coat]
If I saved my money, I could buy a new coat. [I would be able to buy a new coat.]
_________
could is also used for the subjunctive of can in if clauses.
If I can reach that high, I will get the sugar from the shelf for you. [If I am able to reach ...]
If I could reach that high, I would get the sugar from the shelf for you. [If I were able to reach ...]
_________
could cannot be used to show the past success of a past attempt, but could not can be used to show failure.
I tried to catch the ball, and I could. (Wrong.)
I tried to catch the ball, and I was able to. (Correct.)
I tried to catch the ball, but I could not. (Correct.)
I tried to catch the ball, but I was not able to. (Correct.)
_________
could is also used in present time for politeness.
Can you hand me the hammer? Could you hand me the hammer?
(Are you able to hand me ...? Would you be able to hand me ...?)
CJ