The Perfekt of modal verbs follows a special "double infinitive" pattern in German. Instead of using the past participle of the modal, the structure is haben + Infinitiv + Modalverb (infinitive): "Er hat kommen können," "Sie hat gehen müssen," and "Ich habe das Auto kaufen wollen." This rule applies whenever a modal accompanies a dependent infinitive; the past participle of the modal (e.g., gekonnt) is only used when there is no dependent infinitive, as in "Ich habe das gewollt." Word order follows the standard rule: in subordinate clauses, the conjugated verb precedes the double infinitive ("weil er hat kommen können"), not the usual end-of-clause slot.
German has two voices for the passive. Vorgangspassiv (werden + Partizip II) describes the action or process itself: "Die Tür wird geöffnet." Zustandspassiv (sein + Partizip II) describes the resulting state once the action is finished: "Die Tür ist geöffnet." Passives also combine with modal verbs in the form Modal + Partizip II + werden: "Das Problem muss gelöst werden." Two common alternatives to the passive are "sich lassen + Infinitiv" (Das Problem lässt sich lösen = the problem can be solved) and the adjective suffix -bar added to a verb stem (trinkbar = drinkable, machbar = doable, erreichbar = reachable). The Perfekt passive uses sein + Partizip II + worden — never "geworden": "Die E-Mail ist geschrieben worden."
Futur II is built with werden + Partizip II + haben/sein. Its most common role is to express an assumption about a past completed action, often paired with modal adverbs like schon, wohl, or bereits: "Er wird das Buch gelesen haben" (He has probably read the book), "Sie wird schon angekommen sein." A secondary use describes future completion: "Bis morgen werde ich die Arbeit erledigt haben." In both cases the auxiliary verb at the end must match the verb's properties, with sein chosen for verbs of motion or change of state.