Der Zoll kontrolliert das Gepäck, nicht den Reisepass.

Here der Zoll means customs: the customs authority or the customs officers at a border, airport, or checkpoint.

A few useful notes:

• In English, customs looks plural, but in German der Zoll is grammatically singular.
• Zoll can also mean customs duty / tariff in other contexts, but in this sentence it clearly means the border/customs authority.

German main clauses normally put the conjugated verb in second position.

So the structure is:

• Der Zoll = subject
• kontrolliert = conjugated verb
• das Gepäck = object
• nicht den Reisepass = contrast/negation

This is standard German word order for a simple statement.

Also, kontrolliert is the 3rd person singular form of kontrollieren, because der Zoll is treated as singular.

Because Reisepass is the direct object of kontrolliert, so it must be in the accusative case.

• nominative: der Reisepass
• accusative: den Reisepass

Since Reisepass is a masculine noun, its article changes from der to den in the accusative.

That is one of the most important patterns in German:

• masculine accusative: der → den

Because Gepäck is a neuter noun, and for neuter nouns the article is the same in the nominative and accusative:

• nominative: das Gepäck
• accusative: das Gepäck

So both das Gepäck and den Reisepass are objects here, but only the masculine noun shows a visible article change.

In German, Gepäck is usually an uncountable collective noun, much like luggage or baggage in English.

So German normally says:

• das Gepäck = luggage / baggage

Not usually:

• die Gepäcke

If you want to count individual items, German often uses:

• das Gepäckstück = one piece of luggage
• die Gepäckstücke = pieces of luggage

Because nicht is negating that specific part of the sentence.

The meaning is:

• Customs checks the luggage
• not the passport

So nicht is placed right before the thing being rejected or contrasted: den Reisepass.

This is different from negating the whole sentence. Here the sentence is not saying customs does not check anything. It is saying customs checks one thing, not the other.

The comma helps mark a pause and makes the contrast clearer.

It separates:

• what is checked: das Gepäck
• what is not checked: nicht den Reisepass

You can think of it as a shortened contrast, almost like:

• Der Zoll kontrolliert das Gepäck, nicht den Reisepass.
• The customs checks the luggage, not the passport.

The comma makes the correction/contrast easier to hear and read.

Yes. A very common alternative is:

• Der Zoll kontrolliert nicht den Reisepass, sondern das Gepäck.

That also means that customs checks the luggage rather than the passport.

The difference is mainly in presentation:

• das Gepäck, nicht den Reisepass starts with the correct item and then adds the contrast
• nicht den Reisepass, sondern das Gepäck first rejects one option and then gives the correct one

Both are natural.

Because they are nouns, and all German nouns are capitalized.

So in this sentence:

• der Zoll
• das Gepäck
• der Reisepass

That is a basic but very important spelling rule in German.

Yes, but only slightly.

• der Reisepass = passport
• der Pass can also mean passport in everyday speech, but it can have other meanings too in other contexts

So Reisepass is the more precise word here. For learners, it is a good word to remember because it clearly means passport.