Difference between can and be able to

[Saifon]

Test No. incompl/elem-7 "Modals", question 9

When will you ......... come and see us in our new house?

(a) can
(b) be able to
(c) must

Test No. incompl/elem-7 "Modals", answer 9

When will you be able to come and see us in our new house?

Correct answer: (b) be able to

Your answer was: incorrect
When will you can come and see us in our new house?
_________________________

Could you please identify the different between 'can' and ' be able to '? Since I was ensured that I could use ' can' in this sentence, I become surprised to find that my answer is wrong.
Thank you in advance.
Saifon

[Alan]

The auxiliary can is a so called defective verb because there is no infinitive form and no past participle. This means that you have to use other words to replace these missing parts. You use the infinitive after will and so you use the alternative form be able, which acts as the substitute.
_________________
English as a Second Language
You can read my ESL story Sea Expressions

[daltomaciel]

That's ok.
But, in this case:
After the accident he was no longer able to walk.
Could I use "can" or "could" in the case?
How would it be?

[Skrej]

'After the accident he was no longer able to walk.'
'After the accident, he could no longer walk.'

Both are acceptable, yes. However, you might also hear the 2nd version as:
'After the accident, he couldn't walk.' or 'After the accident, he couldn't walk anymore.'

[lost_soul]

In my opinion, the difference kicks in, when you need to use "can" in the past tense and in affirmative sentences, as in:
1. He was able to climb the mountain on that date
2. In his youth, he could climb mountains