Modals. Hmmmm..

"EXCEPTION Conditional with Modal Verbs
There are some special conditional forms for modal verbs in English:
would + can = could
would + shall = should
would + may = might
The words "can," "shall" and "may" cannot be used with "would." Instead, they must be used in these special forms."

I don't understand that.
"If I have a test, I should be studying/study"

I don't understand how "should" = shall + would.
I see it as "should" by itself as an obligation.

I understand that would/could have an implied "if". But why not "should"?