Could = Would be able to? All the time?

Question

I was taught like, one can switch "Could" with "Would Be Able To", in a sense of hypothetical usage, and the future in a past tense like this one below.

I thought you could catch it.

I thought you would be able to catch it.

Q.1) But, can you put "Would Be Able To" into the place of the "Could" which is a past form of can?

When I was young, I could touch a bug with bare hands.

When I was young, I would be able to touch a bug with bare hands.

Q.2) How about the polite version of can?

Could you do this for me?

Would you be able to do this for me?

I'm not worrying about the subtle nuance differences. I'm just worried that they can't be exchanged at all.

Answer 1

For Question 1:

When I was young, I could touch a bug with bare hands.

No, you can't use "would be able to" in this sense — 'would be' becomes 'was' here, if it was the simple past tense:

When I was young, I was able to touch a bug with bare hands.

in the sense of: "I know that I could, because I did touch a bug with my bare hands". (Note: I'd add 'my bare hands' here personally).

If it's past AND hypothetical, then you could use "would have been able to" in place of "could have":

When I was young, I would have been able to climb the Eifel Tower in 10 minutes

As in "I never tried, but I know that I was fit enough to do it". For Question 2:

Would you be able to do this for me?

Yes, this is perfectly natural.

Answer 2

The difference between Q1 and Q2 is that the former doesn't fit your in a sense of hypothetical usage requirement.

As it stands, at least the first part is a declaration of your ability. Rephrased, both parts become more hypothetical, and basically mean the same thing.

When I was young, I could have touched a bug with bare hands.

When I was young, I would have been able to touch a bug with bare hands.