Before I go into the detail, I want to highlight a couple of important things aboutcan.
- Can has only two forms: can (present) and could (past).
- There is no present perfect form.
- There is no infinite (to) form.
Compare:
- I can’t concentrate on work now.
- I haven’t been able to concentrate on work this morning.
And:
- We can finish this report tomorrow.
- We might be able to finish this report tomorrow.
Present Ability and Possibility
When we want to talk about present ability we use both ‘can’ and ‘be able to’, but it’s more common to use ‘can’. ‘Be able to’ sounds more formal.
- Can you finish that report this afternoon? (Are you able to finish that report this afternoon?)
- I can get that done for you, but it won’t be until tomorrow. (I am able to get that done for you, but it won’t be until tomorrow.)
Future Ability and Possibility
When we talk about the future, we can only use ‘will be able to’.
- When I’ve attended that training course, I will be able to create formulas on Excel. (not ‘I can create …’)
- I will be able to finish that report when I get the data from marketing. (not ‘I can finish …’)
Decisions and Future Arrangements
When it’s about decisions, or future arrangements, we use either ‘can’ or ‘be able to’.
- The boss can meet you next Thursday. (The boss is able to meet you next Thursday.)
- Right now I’m busy, but if you give me ten minutes, I can help you out then. (Right now I’m busy but if you give me ten minutes, I’ll be able to help you out then.)
- Rather than us taking the train, can Jonathon drive us? (Rather than us taking the train, is Jonathon able to drive us?)
Past Ability
When we talk about an ability that existed for a while in the past, but which isn’t true now, we use ‘could’ or ‘was/were able to’.
- When I was at uni, I could work all night and still stay awake during lectures (When I was at uni, I was able to work all night and still stay awake during lectures.)
Action Verbs: Ability Related to a Single Event
In this case we only use ‘was able to’ or ‘were able to’.
- We were able to go to the meeting yesterday, even though we were running late, because the traffic was light. (Not ‘We could go …’)
- I was able to finish the report this week. (Not ‘I could finish…’)
- I was able to get a lot done at work yesterday because so many people were away. (not ‘I could get a lot done …’)
Stative Verbs: Ability Related to a Single Event
We use either ‘can’ or ‘was/were able to’ with some stative verbs (e.g. see, hear, feel, taste).
- Were you able to see the new exhibition stand yesterday? (Could you see the new exhibition yesterday?)
- I was able to hear the speaker even though he didn’t use a microphone. (I could hear the speaker even though he didn’t use a microphone.)
- Because I was sitting towards the front, I was able to see the slides clearly. (Because I was sitting towards the front, I could see the slides clearly.)
Negative Statements: single events and actions over a period of time
For negative statements, whether they are single events or actions that took place over a period of time, we use couldn’t’ and ‘wasn’t/weren’t able to’’.
- I wasn’t able to finish the report yesterday. (I couldn’t finish the report yesterday.)
- She wasn’t able to type so quickly before she did that new intensive typing course. (She couldn’t type so quickly before she did that new intensive typing course.)
- We weren’t able to register for the conference. (We couldn’t register for the conference.)
