The verb sollen also corresponds to the English modal verb(s) shall/should: 1. Supposed obligation (“to be supposed to”) that refers to something the speaker understands/thinks that should be happening or have happened: Soll(te)st du nicht schon schlafen? (Aren't you supposed to/shouldn't you be sleeping?)

Sollen und Müssen difference?

According to its definition, sollen is used if something is not completely obligatory, but it would be really disadvantageous for somebody if the opposite happened, whereas müssen is really strict and utilized to determine rules and laws or if something is an inevitable condition for something else.

Sollen V sollten?

The best way to understand the difference between sollen (present) and sollten (subjunctive, called Konjunktiv II in German) is to translate them as "have to" and "should", respectively.