The present time subjunctive of a modal auxiliary verb uses the simple past tense form of the verb, with an Umlaut over the stem vowel, except for sollen and wollen. Subjunctive modal auxiliary verbs are especially important for making polite requests.

Ich möchte zwei Glas Bier. Dürfte ich eine Frage stellen? Sollten wir sie einladen? Könntest du mir helfen?

I would like two glasses of beer. Might I ask a question? Shouldn't we invite them? Could you help me?