Learn Hardcore German: Die Tür ist offen.

Tür is a feminine noun in German. In the nominative case (used for the subject of a sentence), the definite article for feminine nouns is die.

Why is Tür capitalized?

All German nouns—common or proper—are always capitalized. This rule helps you spot nouns in written German.

Why is Tür in the nominative case and not the accusative?

Because die Tür is the subject of the sentence (it’s “doing”—or in this case “being”—something), it takes the nominative case. Accusative would mark a direct object, but ist (a linking verb) doesn’t take an object here.

Why does the verb ist appear in second position?

German main clauses follow the V2 rule: the finite verb must occupy the second position. Here, Die Tür is in first position, so ist comes next.

Why does offen have no ending (adjective ending)?

When an adjective follows a linking verb like sein, it’s a predicate adjective and remains uninflected. You simply describe the state: Die Tür ist offen, not offene or offener.

Could we use the indefinite article eine instead of die?

Yes. Eine Tür ist offen means “a door is open” (some door), whereas Die Tür ist offen means “the door is open” (a specific door known to speaker and listener).

Why don’t we say Die Tür ist geöffnet instead of offen?

Both are possible:

offen is a simple adjective describing the state “open.”
geöffnet is the past participle of öffnen used adjectivally, often highlighting the action that opened it. In everyday speech offen is more direct for the state.

Could we say Die Tür ist auf like we do with windows?

With Fenster you often hear Das Fenster ist auf, because auf acts like an adverb of position. For doors, native speakers prefer offen. Saying Die Tür ist auf would sound odd or marked.

Why use the linking verb sein (ist) instead of the action verb öffnen?

sein + adjective describes a state (the door’s condition). Using öffnen focuses on the action of opening. If you want to talk about someone opening it, you’d say Jemand öffnet die Tür; to describe its current state, you say Die Tür ist offen.

Can I use other adjectives in the same way, for example geschlossen?

Yes. Many adjectives—or past participles used adjectivally—work as predicate adjectives after sein. For example:
• Die Tür ist geschlossen. (The door is closed.)
• Die Tür ist verriegelt. (The door is locked.)