Quote:
Originally Posted by MikeNewYork  I understand your point of view, but I'm not sure I see as much difference between the forms as you do.
When I was a child, I could play the piano
When I was a child, I was able to play the piano.
I see little to no difference.
I can run a mile in 5 minutes.
I am able to run a mile in 5 minutes.
Again, little to no difference for me. |
Can, must, will are the so called
defective verbs because they don't have a past participle (perfect tenses), the infinitive (future tense) or in case of must not even a past tense. This means you need
substitutes to fill the gap. Unfortunately the substitutes have slightly different semantic functions.So it is a kind of
compromise.
Have to for instance cannot replace
must semantically completely.
Can (usually its past form: could) expresses only ability but not whether the speaker has made use of that ability. You might say:
I could escape from prison. It would have been easy for me but I didn't do it.
Be able to by contrast shows the speaker has already made use of his/her ability
I agree with you:
can, could, must, will, would: have far more semantic components than their substitutes: be able to or have to. This is the reason why the substitutes are used as a kind of compromise since they cannot cover the full semantic field.