Could or able to

[oxford2011]

Which of the following sentence is correct
1.Yesterday, I could lift the couch by myself.
2.Yesterday, I was able to lift the couch by myself.
a.Yesterday, I could not lift the couch by myself.
b.Yesterday, I was unable to lift the couch by myself or Yesterday, I was not able to lift the couch by myself.

[Rover_KE]

They are all correct.

[oxford2011]

Thanks for the reply. Also, I need to know can we use could and be able to in same sentence like "Could you be able to walk on the slippery road"

[Rover_KE]

Your suggestion does not work.

'I could get a day off work if you are able to visit us next week.'

I can't think of a sentence using could and be able with the same subject.

Rover

[Tdol]

You could create a context, where it would be possible, though a little forced and unnatural. If someone is unwilling to go somewhere and says they're not free, but you offer them an incentive to go:

A: I'm sorry I don't think I will be able to go.
B: Not even if we fly you first class?
A: Well, on second thoughts, I could possibly be able to go if I were to go first class.

It does sound like something from Yes, Minister, though.

[Verona_82]

Which of the following sentence is correct
1.Yesterday, I could lift the couch by myself.

But the sentence doesn't say whether they actually lifted the couch, doest it?