The modal verb sollen

The modal verb sollen expresses a necessity that is the result of a request/command addressed to the verb's subject by a third party.

This request/command can derive not only from a person but also from something abstract, like social norms. In this case, it expresses a necessity that is attributed to moral duty or regulations.

Differences between sollen and müssen

The modal verb müssen expresses a necessity in general, while sollen expresses a necessity that is the result of a request/command addressed to the subject of the verb.

This means that müssen contains the meaning of sollen, it is, however, not clear that it involves a request/command. If the aspect of "request/command" is important to what is being said, it has to be clarified by the context.

If what is being said contains the aspect of request/command, according to context, sollen can be replaced by müssen.

There is also a difference in the intensity of the necessity. Müssen expresses a strong necessity that cannot be overlooked without consequences.

Differences between nicht sollen and nicht müssen

The modal verb nicht sollen expresses prohibition; nicht müssen expresses lack of necessity.

When sollen is negated, it refers to the infinitive with which it is connected. When müssen is negated, it refers to the modal verb itself.

Prohibition is typically expressed by using the negated form of the modal verb dürfen. Nicht müssen is often replaced by nicht brauchen. These two pairs are differentiated by their intensity.

When the subject is not a person, sollen often expresses a purpose, a goal, a plan, an intention or a function.

In the Konjunktiv II, sollen expresses suggestion, advice or recommendation.

In questions, sollen has a similar function. It expresses indecisiveness and asks for advice. In this case, sollen is in the first person and in the indicative mood. Thereby, sollen can also denote suggestion.

The modal verb sollen gives emphasis to the supposition expressed in a conditional- or a concessive clause. In this case, it is always in the Konjunktiv II Präteritum.

Sollen can also occupy the first position in a sentence; the conditional conjunction is, in this case, omitted.

Sollen is also used in indirect requests and expresses a strong and urgent request/command. It is a function that pertains to the indirect (reported) speech.

The modal verb sollen also expresses, when in the Präteritum of the indicative, the future in the past.

Sollen is used in questions that express doubt. In this case, sollen is in the Konjunktiv II. If the question is introduced by a question word or a pronoun, then sollen can be in the present tense indicative.