1. Modal Verbs

2. Ability or potential:can, could, be able to
Can is ussed to indicate the possession of ability in general, or being in a position, in particular circumstances, to perform the activity denoted by the main verb.
e.g. He can speak Germain well.
e.g. I can (or could*) give him an answer now (later, tomorrow).
* Could function here as a tentative form from can, and may refer to a conditional idea. (I could give him an answer tomorrow if he wants (or wanted) one.)

3. Ability or potential: can, could, be able to
e.g. He can speak Germain well.
e.g. I can (or could*) give him an answer now (later, tomorrow)
Recognoze the distinction between ability in general and ability in specific circumstances
Both sentences refer to a potential performance of the action mentioned, not to an actual performance.

4. Ability or potential:can, could, be able to
e.g. He could speak German fluently when he was younger. - situation in the past - refers to the possession of the ability to speak Germain, not to an actual performance of speaking

5. Ability or potential:can, could, be able to
e.g. I could have given him an answer yesterday. - past tense - ability in specific circumstances at a specific time - doesn’t refer to an actual performance - refers to conditional idea: I could have given him an answer, if he had asked me.

6. Ability or potential:can, could, be able to
If we wish to refer to an actual performance we use form of be able to.
e.g. I was able to give him an answer yesterday

7. Ability or potential:can, could, be able to
Verbs like see, hear, understand come into special category.
e.g. I can/could see quite clearly what you are /were doing.
The ability to see and the performance of seeing are inseparable and in this case the use of could is possible when referring to an actual performance in the past time.

8. Ability or potential:can, could, be able to
The negative form couldn’t necessarily implies non-performance of an action, and may always be used to refer to past time.
e.g. He couldn’t speak Germain well, when I knew him.

9. Ability or potential:can, could, be able to
Can may be used with a future time references.
e.g. I can see you tomorrow.
- the ability is more or less taken for granted

10. Ability or potential:can, could, be able to
In cases where ability will exist only eventually, or where it is dependent on some other event in the future, we use be able with will (‘ll) or shall.
e.g. By the time he finishes his course, he will be able to speak German well.

11. Ability or potential:can, could, be able to
We also use be able when we wish to say that an action was in fact performed in the past.
e.g. After looking at his notes again, he was able to complete the exercise.

12. Ability or potential: can, could, be able to
Can and could lack infinitive and participle forms.
We use be able when we want to express an infinitive or a ‘perfect’ form. Be able is not used in the continuous form.
e.g. He should (ought to) be able to help us.

13. Ability or potential:can, could, be able to
When could + (present infinitive) is used as the tentative form of can, it has a present or future time reference:
a) I can do it for you now if you like it.
b) I cann’t do it immediately, but I could do it tomorrow morning.

14. Ability or potential:can, could, be able to
In conditional sentences, could very often represents ‘unreal’ present.
e.g. If I knew how it worked, I could tell him what to do.

15. Ability or potential: can, could, be able to
In a conditional sentence, could + perfect infinitive expresses unreal past:
e.g. If I had known how it worked, I could have told him what to do. (but I didn’t know, so I couldn’t tell him)

16. Ability or potential:can, could, be able to
Can and could are also used to refer to a general characteristic or quality that may show itself from time to time.
e.g. A house in London can cost a lot of money.
He could be very unpleasant when he was angry.
Neither of these sentences refers to an actual occurence and be able is not used as a substitute for can and could in these sentences.