When describing ability there are two main criteria to consider:
- Referring to a Single Specific Occasion or General Ability.
- Referring to the PAST, PRESENT, FUTURE or HYPOTHETICAL time.
Use Can / Can't for General Ability in the Present.
- I can swim.
- She can't drive a bus.
Use Can / Can't for Specific Ability in the Future.
- I can come on Friday, if you want.
- Can they come to my party?
Note: CAN is used without TO.
- I can fix your bike.
- NOT I can to fix your bike.
Use Could / Couldn't for General Ability in the Past.
- I couldn't write when I was 4.
- She could speak three languages 10 years ago.
Use Could / Couldn't for Hyothetical Ability in the Future.
- We could come later if it isn't raining.
- Couldn't they fix it next week?
Use Could / Couldn't for Specific Ability in the Past, especially with state verbs.
- We could smell the dinner from the other room.
- Couldn't you hear the dog barking?
Use for Hyothetical Ability in the Past.
- We could have gone to America, but we chose not to.
- They couldn't have helped us anyway, they were away that weekend.
Used for Specific Ability in the Past, especially when it was difficult.
- She managed to finish that report on time.
- I didn't manage to do the shopping today, sorry.
- He wasn't able to study because he was ill.
- I was able to read it in Dutch.
Use for Specific Ability in the Future, similar to use of Can.
- I will be able to come if you want.
- Will they be able to deliver it tomorrow?
Use for Hyothetical Ability in the Future, similar to this use of Could.
- I would be able to come if I finish work.
- She wouldn't be able to help because she doesn't speak Danish.
In general the same as CAN, but note the following:
Has the negative form UNABLE, as well as a negative auxiliary.
- They were unable to help. / They weren't able to help.
- She will be unable to come. / She won't be able to come.
Unlike Can, BE ABLE TO can follow another modal.
- She could have been able to do it.
- I must be able to drive my car.