One English sentence, two German ones
The shop is closed. That can mean the shutters are coming down right now, or that the shop has been shut since six. English leaves it open. German has two different sentences, and you have to choose.
Die Tür wird geschlossen.
The door is being closed.
process: someone is closing it
Die Tür ist geschlossen.
The door is closed.
state: it was closed earlier
The auxiliary itself tells you which one it is: werden is the verb of change (to become), sein is the verb of being.
Forms and three stages of one event
The Zustandspassiv is just sein + Partizip II, like a sentence with an adjective. You mostly meet it in the Präsens (Das Auto ist repariert.) and the Präteritum (Das Auto war repariert.). The word worden in the passive Perfekt is what separates the reported event from the state.
Process, event, state
Das Fenster wird gerade geputzt.
The window is being cleaned right now.
the process
Das Fenster ist gestern geputzt worden.
The window was cleaned yesterday.
the finished event, reported
Das Fenster ist geputzt.
The window is clean.
the state now
How to tell which one you need
werden: something is happening
There is an action that takes time. You can add gerade, jeden Montag or in zwei Stunden. The sentence answers what is going on? Naming the doer with von normally goes with werden.
Die Straße wird seit Mai saniert.
The road has been under repair since May.
Das Paket wird von einem Boten gebracht.
The parcel is being brought by a courier.
sein: this is how things stand
Nothing is in progress. The action finished earlier and left a condition. You can add schon, noch or sonntags. The sentence answers what is the situation?
Die Straße ist seit Mai gesperrt.
The road has been closed since May.
Der Tisch ist schon gedeckt.
The table is already set.
Both seit Mai sentences are correct, but they paint different pictures: roadworks going on for months, or a barrier put up in May and still standing. seit does not decide; the auxiliary does.
The state comes from an earlier action
A Zustandspassiv always presupposes that someone did something earlier. Die Tür ist geschlossen implies that someone closed it; Die Tür ist alt just describes the door.
From action to state
Many Partizip II have become ordinary adjectives that no longer suggest a doer: bekannt, erschöpft, begeistert.
Sie ist in der Stadt gut bekannt.
She is well known in town.
Nach der Schicht war ich völlig erschöpft.
After the shift I was completely exhausted.
Where you meet it every day
Signs, shop windows and anyone telling you how things stand use the state passive.
Wir sind ab 9 Uhr geöffnet.
We are open from 9 a.m.
Der Tisch am Fenster ist leider reserviert.
The table by the window is reserved, I'm afraid.
Alle Plätze sind besetzt.
All the seats are taken.
Die Wohnung ist schon verkauft.
The flat has already been sold.
Ich bin in Köln geboren.
I was born in Cologne.
the usual spoken form; Ich wurde in Köln geboren is just as correct
Die A8 wird am Wochenende gesperrt.
The A8 will be closed at the weekend.
process: it is going to happen
