You already know the modal verbs "müssen", "können" and "möchten". Here's another one, "sollen". Like the others, it usually refers to another verb in the sentence, the main verb.

"sollen" (should, ought to) is conjugated without a vowel change and is used predominantly to express demands and recommendations:

Examples:
Sie sollen im Bett bleiben. (You should stay in bed.)
Sie sollen Ihre Medizin nehmen. (You should take your medicine.)

The difference between "sollen" and "müssen"

"sollen" und "müssen" are used similarly, but there are slight differences in meaning. "müssen" is used to express an objective necessity. It sounds more pressing and demanding. "sollen" is mainly a way to reflect someone else's orders.