Wenn er nur daran dächte! Wenn wir uns besser kennten, würden wir mehr Spass haben.

If only he would think of it! If we knew each other better, we would have more fun.

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?

The present time subjunctive of an irregular verb uses as its base the simple past tense verb stem, but with an Umlaut over the stem vowel, if it is a, o, or u. The subjunctive also uses different endings from the simple past for most forms. Thus even verbs with an i, e, or ie stem vowel in the simple past, which can carry no Umlaut, are nevertheless nearly always recognizable as subjunctive forms.