USING COULD, MIGHT AND SHOULD

(a) If I were a bird, I could fly home.
(b) If I could sing as well as you, I would join the opera.
(c) If I'd had enough money, I could have gone to Florida for vacation.

In (a): could fly = would be able to fly
In (b): could sing = were able to sing
In (c): could have gone = would have been able to go


(d) If I don't get a scholarship, I might get a job instead of going to graduate school next fall.
(c) If you were a better student, you might get better grades.
(f) If you had told me about your problem, I might have been able to help you.

In (d): I might get = maybe I will get
In (c): you might get = maybe you would get
In (f): I might have been = maybe I would have been


(g) If John should call, tell him I'll be back around five.
(h) If there should be another world war, the continued existence of the human race would be in jeopardy.

In (g): If John should call indicates a little more uncertainty or doubt than If John calls, but the meaning of the two is basically the same.
In (h): If there should be indicates more uncertainty or doubt than If there were.


Cheers!
Naeem

Source(s):
Understanding and Using English Grammar by Betty Schrampfer Azar