Learn German: Der Schlüssel steckt in der Tür.

In English, we often just say the key 'is' in the door. German strongly prefers specific positional verbs over just using sein. The verb stecken describes something inserted into a tight space, making it the perfect everyday verb for a key in a lock, a plug in a socket, or a card in an ATM.

Why do we use der Tür instead of die Tür if Tür is a feminine noun?

Because in is a two-way preposition and here it describes a fixed location, not a movement. When answering the question 'Wo?' (Where?), two-way prepositions trigger the dative case. The dative form of the feminine die Tür is der Tür.

Does stecken always mean the key is jammed and won't come out?

Not necessarily! While stecken can mean 'to be stuck' (like a car in the mud), in the context of keys, it usually just means 'to be inserted'. It is simply stating where the key is, without automatically implying there is a problem.

How would I use the 'r-' words from this lesson to say 'Put the key in!'?

If you want to express the movement of putting the key inside without repeating the word door, you would use rein (short for hinein). You could say: Steck den Schlüssel rein! If you just wanted to say the key is currently inside (location), you would use drin: Der Schlüssel steckt drin.