Reilen og låskassen sitter bak dørvrideren.

In this sentence:

• reilen = the latch bolt / the bolt
• låskassen = the lock case / the lock body

Both are technical door-lock words, so they are more specific than everyday vocabulary. In normal conversation, many learners will meet lås first, which simply means lock.

Norwegian often uses verbs like sitter, ligger, and står where English would often just use is/are located.

Here, sitter means something like:

• is situated
• is positioned
• is mounted

So Reilen og låskassen sitter bak dørvrideren means that those parts are physically placed behind the handle.

It does not mean they are literally sitting in the human sense.

The ending -en is the usual definite ending for many masculine nouns in Norwegian.

So:

• reil = a latch bolt

• reilen = the latch bolt

• låskasse = a lock case

• låskassen = the lock case

• dørvrider = a door handle/door turner
• dørvrideren = the door handle

So the sentence is talking about specific parts: the latch bolt, the lock case, and the door handle.

In Norwegian, the definite article is usually attached to the end of the noun instead of appearing as a separate word before it.

Compare:

• English: the door handle
• Norwegian: dørvrideren

This is one of the biggest differences from English.
So instead of saying a separate the, Norwegian often builds definiteness into the noun itself.

Dørvrider is a compound noun:

• dør = door
• vrider = turner/handle

Together, dørvrider means door handle or more literally door turner.

Norwegian uses compound nouns very often, much more freely than English. English might write two words, but Norwegian often combines them into one:

• dør + vrider = dørvrider
• lås + kasse = låskasse

So this sentence is a good example of a very common Norwegian pattern.

In the present tense, Norwegian verbs do not change for singular or plural subjects.

So the same form is used with:

• reilen sitter
• låskassen sitter
• reilen og låskassen sitter

Unlike English, Norwegian does not say sit for plural and sits for singular in the present tense. It is just sitter for all of them.

Bak means behind.

So:

• bak dørvrideren = behind the door handle

That is a normal prepositional phrase in Norwegian:

• bak huset = behind the house
• bak døren = behind the door

Nothing is missing. Bak simply takes a noun phrase after it.

The structure is:

• Reilen og låskassen = subject
• sitter = verb
• bak dørvrideren = place/location

So the pattern is:

Subject + verb + place

That is the normal word order for a simple main clause in Norwegian.

Sometimes Norwegian chooses among ligger, står, and sitter depending on how something is positioned or attached.

Very roughly:

• ligger = lies / is lying
• står = stands / is standing
• sitter = sits / is fixed or located in place

Here, sitter sounds natural because the bolt and lock case are installed as parts of the door mechanism. They are not just lying somewhere loosely, and they are not really thought of as standing upright.

So sitter gives the sense of being mounted or fitted in place.

Often they can refer to roughly the same thing in everyday English: door handle.

But there can be slight differences in usage depending on context and hardware type:

• dørhåndtak is a very common general word for door handle
• dørvrider can sound a bit more specific, especially for a handle that you turn

In this sentence, dørvrideren is a natural choice because it refers to the part you turn to operate the latch.

If you changed the nouns to indefinite form, it would be:

En reil og en låskasse sitter bak en dørvrider.

That means something like:

A latch bolt and a lock case are behind a door handle.

But in real usage, the definite form in the original sentence is more natural if we are talking about the specific parts of a particular door or lock.