Morgen übergeben wir dem Vermieter den Schlüssel.

Because it is the first word of the sentence, and the first word in a German sentence is always capitalized.

In the middle of a sentence, morgen meaning tomorrow is normally lowercase:

Wir übergeben morgen dem Vermieter den Schlüssel.

That is different from the noun der Morgen, which means the morning and is always capitalized because all German nouns are capitalized.

Here it means tomorrow.

You can tell from how it is used:

• morgen as an adverb = tomorrow
• der Morgen as a noun = the morning

In this sentence, Morgen is not acting like a noun. It is giving time information, so it means tomorrow.

If you wanted in the morning, you would usually say something like:

• am Morgen
• morgen früh for tomorrow morning

German often puts time expressions early in the sentence, especially in everyday speech and writing.

Putting Morgen first gives it emphasis or makes it the topic:

• Morgen übergeben wir dem Vermieter den Schlüssel.

This is very natural German. It is similar to saying:

• Tomorrow, we hand the key over to the landlord.

You could also say:

• Wir übergeben morgen dem Vermieter den Schlüssel.

That is also correct.

Because German main clauses follow the verb-second rule.

That means the finite verb must be in the second position of the sentence. Here:

1. Morgen = first position
2. übergeben = second position
3. wir = comes after the verb

So even though wir is the subject, it does not have to come first. The important rule is that the conjugated verb stays in second position.

Because dem Vermieter is in the dative case.

With übergeben, the person receiving something is usually in the dative:

• jemandem etwas übergeben = to hand something over to someone

So:

• dem Vermieter = to the landlord

The base form is:

• der Vermieter = the landlord

But in the dative singular masculine, der changes to dem:

• nominative: der Vermieter
• dative: dem Vermieter

Because den Schlüssel is in the accusative case.

It is the thing being handed over, so it is the direct object.

The noun is:

• der Schlüssel = the key

In the accusative singular masculine, der changes to den:

• nominative: der Schlüssel
• accusative: den Schlüssel

The noun Schlüssel itself does not change here; only the article changes.

The cases tell you that:

• dem Vermieter is dative = the receiver
• den Schlüssel is accusative = the thing transferred

So the sentence structure is:

• wir = subject
• dem Vermieter = indirect object / recipient
• den Schlüssel = direct object / thing being handed over

This is one of the big advantages of German case marking: the articles show each noun’s role.

Because German often does not need a preposition here.

The verb übergeben can take two objects directly:

• jemandem etwas übergeben

Literally:

• someone-DATIVE something-ACCUSATIVE hand over

English often uses to:

• hand the key over to the landlord

But English can also do something similar without to:

• hand the landlord the key

German works more like that second pattern.

No, not in this meaning.

Here übergeben is an inseparable verb, so it stays together:

• Wir übergeben den Schlüssel.

In the perfect tense, it also does not get an extra ge-:

• Wir haben dem Vermieter den Schlüssel übergeben.

That is an important thing to remember, because many German verbs with prefixes are separable, but this one is not.

Yes, within limits.

These are all possible:

• Morgen übergeben wir dem Vermieter den Schlüssel.
• Wir übergeben morgen dem Vermieter den Schlüssel.

Both mean the same basic thing.

However, the order of the two objects matters stylistically. With two full noun phrases, dative before accusative is usually the most natural order:

• dem Vermieter den Schlüssel

You may also hear other orders for emphasis, but the version in your sentence is the most standard and natural.

Vermieter means landlord or lessor, literally the person who rents something out.

It comes from the verb vermieten, meaning to rent out.

Related words:

• der Vermieter = male landlord
• die Vermieterin = female landlord
• der Mieter = tenant
• mieten = to rent
• vermieten = to rent out

So dem Vermieter means you are giving the key back to the person who owns or rents out the property.

Because they are nouns, and German capitalizes all nouns.

So:

• der Vermieter
• der Schlüssel

This is a standard German spelling rule and one of the first things learners notice.

By contrast, words like morgen meaning tomorrow are not normally capitalized unless they begin the sentence.

It would usually be:

Morgen haben wir dem Vermieter den Schlüssel übergeben.

or, if you mean it already happened:

Wir haben dem Vermieter den Schlüssel übergeben.

Notice:

• the helping verb haben is in second position
• übergeben goes to the end
• there is no extra ge-

This is useful because it confirms again that übergeben is inseparable in this meaning.