A simple dictionary review of shall and should would keep most writers away from using these words. So many times people have told me that shall means obligated and should means optional. They are right and, unfortunately, so wrong.
In grammar, should is nothing more than the past tense. To express the past tense of I shall leave soon, I would say I told her I should leave soon. So no difference, right?
After mere grammar, things get complicated. In legal documents shall does mean obligated, as in The contractor shall complete the job by August 31. After law, the differences between shall and should depend on context. You might say, I shall go to the beach today, meaning you plan to go, but are not obligated to go, to the beach. On the other hand, the spiritual We Shall Overcome is undoubtedly an imperative.
Now let's look at should. If I say as your subordinate, We should start the job now, I am actually asking for your permission, so no obligation to you there. But if you, again as my manager, said the same sentence, I am taking your meaning as a must.
Better to make your meaning precise by using a word like must when you mean obligation, and may, might, or can when you mean possibility.
