Talking about abilities!

CAN and CAN'T /CANNOT - General Ability.

BE ABLE TO - General Ability, more formal.

REMEMBER: Can is POLYSEMOUS! (Ability, Possibility, Permission).

MANAGE TO - To do with great difficulty

COULD / COULDN'T - As a kid they couldn't swim very well.

WAS/WERE ABLE TO - As a kid they weren't able to swim very well

For Most Verbs, use 'WAS/WERE ABLE TO'. e.g.: Yesterday my mom was able to bake a wonderful cake

For PERCEPTION and MENTAL PROCESS verbs, use COULD:

I couldn't hear you over the loud music. I could understand the teacher yesterday, but not today.

Use MANAGED TO for contextual difficulty.