Many students struggle with the difference between the two modal verbs müssen and sollen. The reason is both express obligation but in a different way.
This modal verb, used without negation, corresponds to the English “must/have to + Infinitive” (or “need to”): Ich muss gehen. (I must/have to go.)
The only difference is that the English “must” is a defective verb, meaning it has no conjugation in all the tenses while the German müssen does.
The meaning of müssen is own/proper obligation, i.e. if you want or need to get or do something, you must do something for it. Example: Ich möchte die Prüfung bestehen, also muss ice lernen. (I’d like to pass the exam, thus I have to study.)
It means also you need (to do) something, examples: Ich muss diese Prüfung bestehen! (I need to pass this exam [in order to pass to the next level]). / Ich muss zur Toilette [gehen]. (I need to go to the toilet.)
Attention: When müssen is used with negation (muss nicht/kein), it does not correspond to the English “must not” because it does not express prohibition. It expresses only that there is no obligation (to do something). In this sense, it corresponds to need not: Du musst deine Schuhe nicht ausziehen. (You needn’t take off your shoes.)
This modal verb is very special in the German language. If you want to translate it to other languages, in most cases you have to use a complete expression or rephrase it.
The meaning of sollen is obligation by others, i.e. another person (your parents, your teacher, your doctor…) tells/wants you to do or not to do something.
Notice: It does not matter who the other person is (the person needn’t be mentioned) because this verb implies already another person told you so.
Positive: Du sollst aufpassen.([Your teacher/parents told] you to pay attention.) | Was soll ich sagen? (What do you want me to say?)
Negative: Du sollst die anderen night ärgern. (You [were told] not to bother the others.)
In this last example, you can observe an important connotation that sollen has:
Moral obligation: something that is generally accepted to be good (like helping others) or bad (bothering other people). For this reason, several of the Ten Commandments are formulated with the verb sollen in German.
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?)
2. Advice (used in Konjunktiv II in German: sollte): Du solltest etwas mehr Sport treiben/weniger rauchen/gesünder essen. (You should do more sport/smoke less/eat healthier.)
3. Asking another person if s/he wants you to do something (“shall I…”): Soll ich das Fenster aufmachen? (Shall I open the window?)
4. Propositions: Wir sollten mal wieder ins Kino gehen. (We should go to the cinema again.) | Sollen/Wollen wir etwas zusammen machen? (Let’s do something together.)
Infinitive after indirect questions
In German we can’t combine an indirect question with an infinitive like in English or Spanish:
I don’t know what to say. | No sé qué decir.
Also in this case, we use the verb sollen in German and conjugate it (in final position because it is an indirect question):
Ich weiß nicht, was ich sagen soll.