Learn Hardcore German: Die dritte Tür ist grün.

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

Why does dritte end with -e?

When you use an ordinal number (first, second, third…) as an adjective before a noun, it takes an adjective ending that agrees with the noun’s gender, case, and number. For a feminine noun (Tür) in the nominative singular with a definite article (die), the weak ending is -e, giving you die dritte Tür.

How would the ending change if Tür were masculine or neuter?

It depends on gender and case:

Masculine nominative: der dritte Mann (ending -e)
Neuter nominative: das dritte Kind (ending -e)
Plural nominative (any gender): die dritten Türen (ending -en)

Why is grün not grüne?

In Die dritte Tür ist grün, grün is used predicatively (after the verb sein). Predicate adjectives do not take endings, so you leave it as grün. If you put the adjective before the noun (attributively), you would get die grüne Tür.

What is the difference between an ordinal number and a cardinal number in German?

A cardinal number tells you “how many” (eins, zwei, drei…).
An ordinal number tells you “in which order” (erste, zweite, dritte…).
Here dritte means “third,” indicating order.

Why is the noun Tür capitalized?

German always capitalizes all nouns, no matter where they appear in a sentence. That’s why Tür has a capital T.

Could you say dritter Tür or drittes Tür instead?

No. The adjective ending must match the noun’s gender (feminine), case (nominative), and the presence of a definite article (die). That pattern requires the weak ending -e, so only dritte is correct.

What case is die dritte Tür in?

It’s nominative, because it’s the subject of the sentence (“the third door” is what is green).

How would you ask “Which door is green?” in German?

You would say Welche Tür ist grün? Here welche is the interrogative adjective “which,” and like any adjective before a feminine noun in nominative singular with die, it takes the weak ending -e.

How do you pronounce dritte Tür?

dritte is pronounced [ˈdʁɪ.tə] (trilled/uvular r, short i, final -ə like the “a” in “sofa”).
Tür is [tyːʁ] (long ü as in French tu, soft final -r).