Ich drehe den Schlüssel in der Tür.

Because Schlüssel is the direct object of the verb drehen.

• der Schlüssel = nominative masculine singular
• den Schlüssel = accusative masculine singular

In this sentence:

• Ich = subject
• drehe = verb
• den Schlüssel = the thing being turned

So German uses the accusative case here.

Because this phrase describes location, not movement toward a destination.

A very common German rule is:

• in + dative for where
• in + accusative for where to

Here, the key is being turned while it is in the door, so German treats that as a location:

• in der Tür = in the door / in the lock of the door

If the sentence were about putting something into the door, you would expect in die Tür.

Here der is not masculine nominative. It is the dative feminine form of die.

The noun is:

• die Tür = the door

In the dative case, die Tür becomes:

• der Tür

So:

• nominative: die Tür
• accusative: die Tür
• dative: der Tür

This is a very common point of confusion, because der can represent different cases and genders.

Drehe is the 1st person singular present tense of drehen.

The verb conjugates like this:

• ich drehe
• du drehst
• er/sie/es dreht
• wir drehen
• ihr dreht
• sie/Sie drehen

So Ich drehe means I turn or I am turning.

It can mean either one, depending on context.

German does not usually have a separate everyday form for the English progressive (am turning). The normal present tense often covers both ideas:

• Ich drehe den Schlüssel.
  • I turn the key.
  • I am turning the key.

Context tells you which meaning is intended.

Here drehen means to turn in the physical sense: to rotate something.

Examples:

• den Schlüssel drehen = to turn the key
• den Kopf drehen = to turn one’s head
• das Rad drehen = to turn the wheel

It is a very normal verb for rotating an object.

It is understandable, but many native speakers would often say:

• Ich drehe den Schlüssel im Schloss.

That means I turn the key in the lock, which is a bit more precise and idiomatic in many situations.

Why?

• die Tür = the door
• das Schloss = the lock

So if the focus is specifically on where the key is inserted, im Schloss is often the more natural choice.

Still, in der Tür is not wrong if the meaning is clear.

The nouns are:

• der Schlüssel = the key (masculine)
• die Tür = the door (feminine)

Gender matters because the articles change with the case.

In this sentence:

• der Schlüssel → den Schlüssel because it is accusative
• die Tür → der Tür because it comes after in in a location sense, so it is dative

German articles carry a lot of grammatical information, so learning noun gender is very important.

Yes, parts of the sentence can move, but the conjugated verb still stays in the second position in a normal statement.

Standard order:

• Ich drehe den Schlüssel in der Tür.

You could also say:

• In der Tür drehe ich den Schlüssel.

That version puts more emphasis on in der Tür.

But the verb drehe must remain in second position in a main clause.

Because German articles change according to gender, number, and case.

English uses the everywhere:

• the key
• the door
• turn the key in the door

German changes the article:

• der Schlüssel / den Schlüssel
• die Tür / der Tür

So even though English keeps the, German does not.

A rough pronunciation guide is:

• Schlüssel ≈ SHLÜS-sel

A few helpful points:

• sch sounds like English sh
• ü is a front rounded vowel that English does not really have
• the stress is on the first syllable: SCHLÜS-sel

If you cannot produce ü yet, a rough learner approximation is better than avoiding the word, but it is worth practicing because u and ü distinguish different words in German.

Yes. That would still be a correct sentence.

• Ich drehe den Schlüssel. = I turn the key.

Adding in der Tür gives extra information about where the key is being turned.

So the longer sentence is just more specific.

No. In is a two-way preposition (Wechselpräposition), which means it can take either:

• dative for location
• accusative for direction/movement toward something

Compare:

• Der Schlüssel ist in der Tür. = The key is in the door. (location → dative)
• Ich stecke den Schlüssel in die Tür. = I put the key into the door. (direction → accusative)

In your sentence, it is dative because the phrase describes where the key is while it is being turned.