Could is also used in unreal conditional ("if") sentences.
of this kind refer to situations that are
impossible, contrary to fact--in other words,
unreal situations. In them, could is used
in place of can,
but the time is present
or future, not past.
If I could solve this problem, I wouldn't need your help.
( = I need your help because
I can't solve this problem.)
If Julia could afford to buy a car, she wouldn't have to
to work by bus.
( = Julia has to go to work
by bus because she can't afford
to buy a car.)
If he could
start all over again, he'd do a better job.
( = He can't start all over
again, so he won't do as well
with the job as
he would like.)
If she couldn't help you, she would tell you.
( = She won't tell you that
she can't help you because she can.)
In all the sentences above,
the time for both the "if" clause
the result clause is present or future, not past, because
in conditional sentences, a difference in tense (above,
and time (above, present or future) is a
kind of "signal"
used to show that the
situations are hypothetical, contrary