We normally use 'can' and 'could' in general meaning and 'be able to' in specific situations with the meaning 'manage'. It's clear. But instead of 'be able to' in past negations we use only 'could'. Is that right? So we is it incorrect to say "He was not able to cross the ocean in a little boat"???
What is the difference between 'can' and 'am, is, are able to' in the Present?
Nata wrote:So we is it incorrect to say "He was not able to cross the ocean in a little boat"?
Prior to internet epoch, the answer to an item of English use would depend on the say of English native speakers. But now there may be another source to know the answer. I searched exact match for "He was not able to cross", and knew many people use it.
There will be of course even more people using "He was not able to" (without 'cross').
There are much more people using "was/were not able to" (without 'He').
Therefore, your example "He was not able to cross the ocean in a little boat" is quite acceptable.
-----------------------
Nata wrote: What is the difference between 'can' and 'am, is, are able to' in the Present?
In expressing ability or 'manage', there will be little difference between Can and Be Able To.
Ex: He can / is able to come tomorrow at 2.15.
Ex: Yesterday he could / was able to come on time.
However, since Can may be also a modal auxiliary verb, it expresses an uncertainty – even with it past form Could and with a future time adverbial:
Ex: Highway 89 could reopen tomorrow, authorities believe.
Ex: Rates could rise next month.
== Why I call these modal auxiliary verbs the Future Tense is explained in my humble website englishtense.com.
But Be Able To is not a modal verb and its past form is not suitable to express such an uncertainty to the future:
Ex: ?Highway 89 was able to reopen tomorrow, authorities believe.
Ex: ?Rates were able to rise next month.
Modal auxiliary verbs are difficult to explain, and the above is tailored mainly to meet your questions.
