To express that someone has the power or skill to do something, can and be able are used.
- I can't help you.I am busy.
- I'm unable to help you.
- When I was young I was able to earn my living pretty well; I could work hard. Now I can't. I'm too old.
- I can stand on my head for five minutes.
- Can you speak Arabic?
- Yes, I can.
In the present:
Express ability in the present as follows:
- I can speak good English.
- I can't stand on my head.
In the past
Express ability in the past as follows
- I was unable to visit him.
- I couldn't eat at all when I was ill.
In the future
Express ability in the future as follows
- I will be able to buy a house when I get a good job.
- The teacher can assist you after class if you have any questions.