"Shall" has two uses; telling someone they will do something: "You shall pay your taxes." (In this case, "shall" means "will," with an implied order from the speaker.) Or saying that something is going to happen in the future: "The mountains shall fall." (In this case, "shall" means "are going to") "Shall" is not used very much in common English; we generally use "have to" ("You have to pay your taxes") or "will" ("The mountains will fall.") "Should" means you are giving someone advice on a course of action to take. If I want you to ride your bike to work, I can say, "You should ride your bike to work." It doesn't necessarily imply giving an order or authority like "shall" does. "Should" is not a past tense of "shall;" they are not forms of the same word. Hope that helps!

shall = intention should = obligation