Ich muss - wir müssen
Ich mag - wir mögen
Ich kann - wir können
Ich darf - wir dürfen
Why are the singular persons so different from the infinitive form, for these verbs? It's clearly not umlaut, at least not in the usual sense. (It could be for "muss/müssen", but not for the other three.) Is there an explanation, analogous for example to "i-mutation" for "fahren/fährst"?
In Dutch ("kan/kunnen") and Swedish ("kan/kunna") there is something similar, so I guess it's something quite "old", linguistically speaking. But what is it?
Vielen Dank!