Can/Could vs Be Able To
Can/could and ‘be able to’ are two different usages made in the English languages and they both carry different senses with them. Can is used to indicate the idea of possibility as in the sentence ‘I can do the work’. This only gives the idea that ‘it is possible for me to do the work’. The same is true even in the case of ‘could’.
On the other hand ‘could’ is generally used as a past tense form of ‘can’ as in the sentence ‘I could go’. This sentence means that ‘it was possible for me to go’. Hence the primary usage of both can and could consists in indicating the sense of possibility.
‘Could’ is sometimes used when we want to say that we had the ability to do something, but we did not try to do it as in the sentence, ‘I could have married anybody I liked’. The sense of the sentence is that ‘I had the ability to marry but I did not try after all’.
Sentences with ‘can’ often give information about the behavior or the nature of the object or the place it describes as in ‘The city of Brisbane can be very hot during summer’. Generally ‘could’ is used to talk about the past as in the sentence ‘My brother could be very helpful at time’.
On the other hand ‘be able to’ indicates general ability as in the sentence ‘You are certainly able to dance’. It should be noted that the usage of ‘be able to’ is of course not very common like ‘can’ and ‘could’. On the other hand ‘be able to’ is used to talk about future ability as in the sentence ‘One day you will be able to do it’. This particular sentence is a lot different from the sentence ‘One day you can do it’.