How do German modal verbs work?
The six modals — können, müssen, dürfen, sollen, wollen, mögen — are conjugated in second position while the main verb goes to the end of the clause as a bare infinitive. The trap is negation: nicht müssen means do not have to, while nicht dürfen means must not, which is the opposite of what the English mapping suggests.
The main verb goes to the end
The modal takes the normal verb position and the verb it governs drops to the end of the clause in the infinitive, with no zu. This creates the bracket that characterises German sentence structure.
Ich kann Deutsch sprechen.
I can speak German.
Wir müssen morgen früh aufstehen.
We have to get up early tomorrow.
…, weil ich arbeiten muss.
…because I have to work.
Subordinate clause: modal goes last of all.
What each one means
Können is ability or possibility, müssen necessity, dürfen permission, sollen obligation imposed by someone else, wollen intention, and mögen liking — usually in its subjunctive form möchte for polite requests.
Ich darf hier parken.
I'm allowed to park here.
Permission, not ability.
Ich soll ihn anrufen.
I'm supposed to call him.
Someone else's instruction.
Ich möchte einen Kaffee.
I'd like a coffee.
The polite form of mögen.
The negation trap
This is the one that changes meaning rather than just sounding foreign. Nicht müssen removes the obligation; nicht dürfen imposes a prohibition. English must not maps to nicht dürfen, and translating it as nicht müssen tells people the opposite of what you intend.
Du musst nicht kommen.
You don't have to come.
No obligation.
Du darfst nicht kommen.
You must not come.
Prohibited.
Common mistakes
Ich kann sprechen Deutsch.
Ich kann Deutsch sprechen.
The infinitive goes to the very end, after its own objects.
Du musst nicht rauchen. (meaning: you're forbidden)
Du darfst nicht rauchen.
Nicht müssen means no obligation. For a prohibition you need nicht dürfen.
Ich kann zu schwimmen.
Ich kann schwimmen.
Modals take a bare infinitive with no zu.
Frequently asked questions
Why do modals look irregular in the singular?
They are a historically distinct verb class and change their stem vowel in the singular — ich kann, du kannst against wir können. The ich and er forms also take no ending, which is unique to modals.
What is the difference between sollen and müssen?
Müssen is internal or absolute necessity; sollen is an obligation that comes from somebody else. Ich muss arbeiten is I have to work; ich soll arbeiten implies someone has told me to.
How do modals work in the perfect tense?
They take a double infinitive rather than a participle: ich habe arbeiten müssen, not ich habe gemusst. In practice the simple past — ich musste — is far more common in speech and writing alike.