[STCrowley]

First things first, Majid:

I think you mean "I would appreciate IT greatly. . ."

And as for the -ing verbs after modals, I think you might be confusing yourself. "You should swim" and "you should be swimming" are exactly the same. . . except that the second form is the progressive. Use it whenever you'd normally use the progressive. (If you want, I can send you a link to a series of worksheets I made on the progressive, but it's quite a bit.)

Or did I not understand your question, exactly?

-Toby