The reflexive "å beklage seg" does exist in Norwegian as well, and means to express distress or dissatisfaction with something.
However, I'd advise you to use it sparingly. "Å klage" is indeed the more common, and simpler translation of "to complain".
"å måtte" is the infinitive, while "måtte" on its own is the past tense.
I don't understand why it is Å måtte instead of å må? Isn't måtte the past form? In fact I don't understand any of the past forms of modal verbs since most of them don't seem to actually be used for the past
"Å måtte" is the infinitive, while "måtte" on its own is the past tense.
å måtte spise = to have to eat
jeg må spise = I have to eat
jeg måtte spise = I had to eat