Learn English – “must to be” vs. “must be”

SUPPLEMENTARY to TRomano's answer.

We use could for general ability. But if you want to say somebody did something in a specific situation, use was/were able to, or managed to (not could).

This does not accurately describe the use of these terms.

Both CAN and BE able to may be used in either present or past tense to designate ‘general ability’:

John {can / is able to} bench-press 300 pounds.
In his prime, John {could / was able to} bench-press 300 pounds.

And both may be used in either present or past tense to designate ability on a specific occasion either at the time you are speaking of or subsequent to that time:

You’re not very well hidden; I {can / am able to} see you! ... The construction with can is if anything a little more likely here.
She wasn’t very well hidden; I {could / was able to} see her.

Dr. Fisher {can / will be able to} see you this afternoon.
She said that Dr. Fisher {could / would be able to } see me that afternoon.

The difference between the two expressions CAN and BE able to arises when we are speaking not of the prospective (forward-looking) ability to do something but of the retrospective (backward-looking) fact of having successfully done something. A past achievement calls for BE able to:

Despite pitching on an ankle that was literally stitched together, Schilling was able to lead the Red Sox past the Yankees and Cardinals to Boston's first championship in 86 years.

As for manage to—this is used only to speak of achievement, not ability. As TRomano says, it emphasizes the fact of overcoming an obstacle.

You need the permission of the compiler for things to be accepted, and the compiler is denying you that permission. "May I do this?" "You may not."

May is just as strong as must. It's only weak in the positive sense, where once you're granted permission, you're not obligated to do it.