"Where are you, usually?"
Translation:Hvor plejer I at være?
Well there's a conjugated form of to be in the English version as well (are). In the Danish construction, there are two verbs: at pleje and at være.
The first verb, pleje is conjugated in the present => plejer
This second verb, therefore, is left as an infinitive => at være
I think your problem here is that there isn't an English equivalent of at pleje, since the adverb usually is used for that meaning.