3. müssen (to have to, must)
Present: Ich muss dort Deutsch sprechen. I have to speak German there.
Past/Preterite: Er musste es nicht tun. He didn't have to do it.
Pres. Perfect/Perfekt: Wir haben mit der Bahn fahren müssen. We had to go by train.
Future/Futur: Sie wird morgen abfahren müssen. She will have to depart tomorrow.
Subjunctive/Konjunktiv: Wenn ich müsste... If I had to...
NOTE: For all modals with umlauts, the simple past (preterite/Imperfekt) has no umlaut, but the subjunctive form always has an umlaut!
Sample Idiomatic Expressions:
Ich muss nach Hause. I have to go home.
Muss das sein Is that really necessary
So müsste es immer sein. That's how it should be all the time.