Impersonal verbs have only an "it" form and an infinitive; their real subject is an infinitive or a clause: oportet, decet, constat, attinet, interest, and passives like detur.
oportet "it is right, one ought" + infinitive, accusative and infinitive, or subjunctive: meminisse … oportet "one must remember", advertas oportet "you must notice". decet "it befits" + accusative of the person: prout Romanum, virumque decet "as befits a Roman and a man". necesse est "it is necessary".
constat "it is agreed, it is clear" + accusative and infinitive: momentaneum tantum esse constat "it is clear that it is only momentary". But constat with an ablative or ex is personal, "consists of": constat id omne caruncula (2.2).
attinet "it concerns, it matters": quid attinet …? "what is the point of …?". interest "it makes a difference": nihil interesse "that it makes no difference". Also licet "it is allowed", placet "it seems good", accidit, fit "it happens", restat "it remains".
A passive verb can be impersonal: detur "it is granted (to someone)", itur "people go". Supply "it" or "one" as the English subject.