De sleutelpassage ligt boven de valvloer.

Because both nouns in the sentence are common-gender singular nouns:

• de sleutelpassage
• de valvloer

In Dutch, the definite article is either de or het. Here, both nouns take de, so you get de ... de ....

English uses the for all nouns, but Dutch splits this into two articles.

Dutch nouns are assigned to either:

• common gender → de
• neuter gender → het

Sleutelpassage is a de-word, so it must be de sleutelpassage.

A useful thing to know about compounds in Dutch: the last part usually determines the grammatical gender of the whole word. Since passage is a de-word, sleutelpassage is also a de-word.

The same idea applies to valvloer: the final part vloer is a de-word, so the whole compound is de valvloer.

Yes. Dutch very often combines nouns into a single compound word where English might use two words or a phrase.

So:

• sleutel = key
• passage = passage/section/corridor, depending on context
• sleutelpassage = literally something like key passage

This is completely normal in Dutch. In English, learners are often tempted to split such words up, but in Dutch they are usually written as one word.

In compounds, sleutel- often means key, in the sense of:

• important
• central
• crucial

So it often has the same figurative meaning as English key in expressions like:

• key point
• key moment
• key area

It does not have to mean an actual physical key every time.

Dutch often uses position verbs where English simply uses to be.

Here, ligt comes from liggen, which literally means to lie, but very often it means:

• to be located
• to be situated

So De sleutelpassage ligt boven de valvloer can be understood as saying that the passage is located above the trap floor.

This is very normal Dutch. English often says is, but Dutch prefers more specific verbs like:

• liggen = lie / be located
• staan = stand
• zitten = sit / be in
• hangen = hang

Yes. That is one of the big differences between Dutch and English.

In Dutch, liggen is often used for places, areas, buildings, and objects whose position is thought of as horizontal or simply as located somewhere.

Examples:

• Amsterdam ligt in Nederland.
  Amsterdam is in the Netherlands.
• Het boek ligt op tafel.
  The book is on the table.
• De tunnel ligt onder de weg.
  The tunnel is under the road.

So in your sentence, ligt does not necessarily mean the passage is physically sprawled out in a dramatic way. It often just means is situated.

Boven means above or over in a spatial sense.

So boven de valvloer means above the trap floor.

A useful comparison:

• boven = above, higher than, not necessarily touching
• op = on, on top of, usually with contact

So:

• boven de valvloer = above the trap floor
• op de valvloer = on the trap floor

That distinction is important.

This is standard Dutch main-clause word order:

• De sleutelpassage = subject
• ligt = finite verb
• boven de valvloer = adverbial/prepositional phrase

Dutch main clauses normally put the finite verb in second position. This is often called the V2 rule.

So the structure is:

Subject + verb + rest

You could also move another element to the front for emphasis, and the verb would still stay in second position:

• Boven de valvloer ligt de sleutelpassage.

That still means the same thing, but it puts more focus on boven de valvloer.

Yes.

It is made from:

• val = fall
• vloer = floor

So valvloer is literally something like fall-floor or trap floor, depending on the context.

Again, Dutch commonly builds words this way. The final part, vloer, is the main noun, and the first part, val, specifies what kind of floor it is.

Ligt can be tricky for English speakers.

A rough guide:

• lig- has the Dutch g/ch sound, which is not the same as an English g
• the vowel is short, roughly like the i in sit
• the final t is pronounced

A very rough approximation is something like likht, but with a Dutch throat sound for gh/kh.

If pronunciation is your goal, the hardest part is the Dutch g in ligt.

You might be understood, but ligt is more natural Dutch if you are talking about location.

Using is is sometimes possible in Dutch, but it often sounds less idiomatic when Dutch prefers a position verb.

So:

• De sleutelpassage ligt boven de valvloer. → natural
• De sleutelpassage is boven de valvloer. → understandable, but less natural in many contexts

A more formal alternative would be:

• De sleutelpassage bevindt zich boven de valvloer.

That means The key passage is located above the trap floor, but it sounds more formal than ligt.