Hi. Would you say there is no difference between the modal verb "could" in the negative (as in "could not") and the verb form "was not able to" when mentioning an action in a specific moment in time or some extended period of time in the past?

Example for a specific time in the past?

He could not/was not able to lift the chair in the living room yesterday.

Example for some extended period of time in the past?

He couldn't/was not able to lift a forty-pound chair when in his twenties.
Though I don't think the difference is always clear-cut, I think I'd still tend to go with "couldn't" to refer to a general ability (or in this case, inability).

- He couldn't move the sofa. He's not strong enough to do it without some help.

- He wasn't able to move the sofa out of the livingroom yesterday, but he told me he would have time to do it tomorrow for sure.
AnonymousWould you say there is no difference ...
I can't possibly think of all the sentences that might be possible with those words, so no, I wouldn't say there is no difference. There may be a difference I'm not aware of. Maybe it's just that I can't think of a counterexample at the moment. In general, though, I don't sense a great deal of difference, if any, in that specific context.

I looked all over for my wallet, but I [couldn't / wasn't able to] find it.
Even at age 18, Karen's brother still [couldn't / wasn't able to] read.
Susan and Sally tried several times to lift the refrigerator, but they just [couldn't / weren't able to] do it.
Walter [couldn't / wasn't able to] speak Russian five years ago, but he's quite fluent now.

If it's of any interest to you, I tend to use couldn't in preference to wasn't/weren't able to in all those sentences above.