El pasadizo está bajo la trampilla.

Because this sentence is talking about location: where the passage is.

In Spanish, estar is normally used for location:

• El pasadizo está bajo la trampilla. = The passage is under the trapdoor.

Ser is used for identity, description, origin, etc., not for where something is located in space.

So:

• es = it is / it’s, in the sense of identity or characteristics
• está = it is / it’s, in the sense of location or temporary state

Pasadizo means passage, passageway, or sometimes corridor, often a narrow one. In some contexts it can sound like a secret passage or a hidden connecting passage.

It is a masculine noun, which is why the sentence has el pasadizo.

Because Spanish nouns have grammatical gender.

• pasadizo is masculine → el pasadizo
• trampilla is feminine → la trampilla

The articles have to match the gender of the nouns:

• el for masculine singular
• la for feminine singular

This does not mean the objects themselves are biologically male or female; it is just a grammatical category.

Trampilla usually means trapdoor, hatch, or a small door/opening in a floor, ceiling, or wall.

In this sentence, la trampilla is the thing above the passage.

In Spain, trampilla is a very natural word for trapdoor/hatch.

Here, bajo means under or below.

So:

• bajo la trampilla = under the trapdoor

In this sentence, bajo is functioning as a preposition.

It can also be an adjective in other contexts, meaning low or short:

• un techo bajo = a low ceiling
• un hombre bajo = a short man

But in your sentence, it means under.

Yes. That is a very common alternative.

• El pasadizo está bajo la trampilla.
• El pasadizo está debajo de la trampilla.

Both mean essentially the same thing: The passage is under the trapdoor.

A useful difference for learners:

• bajo + noun = under
• debajo de + noun = underneath / under

Debajo de is often felt to be a little more explicit for physical position, and many learners find it easier because it maps neatly to underneath.

Because bajo as a preposition does not need de.

So you say:

• bajo la trampilla

But with debajo, you normally need de:

• debajo de la trampilla

Compare:

• Está bajo la mesa.
• Está debajo de la mesa.

Both are correct, but the structure is different.

The accent mark shows both pronunciation and distinguishes it from other words.

• está = is / is located
• esta = this (feminine), as in esta trampilla = this trapdoor

So the accent is important:

• está = verb form from estar
• esta = demonstrative adjective

The word order is the normal, neutral order in Spanish:

Subject + verb + location

• El pasadizo
  • está
    • bajo la trampilla

Spanish can sometimes change word order for emphasis, but this version is the most straightforward and natural one for a learner.

For example, Bajo la trampilla está el pasadizo is possible, but it sounds more marked or literary.

Yes, in a normal sentence like this, you usually do.

Spanish uses definite articles more often than English does. English might sometimes omit them in places where Spanish keeps them, but here Spanish naturally says:

• El pasadizo
• la trampilla

Leaving them out would usually sound wrong:

• ❌ Pasadizo está bajo trampilla

So the full noun phrases are needed.

In most of Spain, ll is pronounced like a y sound.

So trampilla sounds approximately like:

• trahm-PEE-ya

And the stress is on -pi-:

• tram-PI-lla

Depending on the region, the exact sound may vary a bit, but for most learners, pronouncing ll like English y is a good approach.

A helpful approximate pronunciation is:

el pah-sah-DEE-tho es-TAH BAH-ho lah trahm-PEE-ya

Notes for Spain:

• z in pasadizo is often pronounced like the th in thin in most of Spain
• j is a strong throaty sound, but there is no j in this sentence
• the stress falls on:
  • pasadizo
  • está
  • trampilla

Yes, very possibly.

Pasadizo often suggests a narrow passageway, and in many contexts it can sound like a hidden or secret passage. The exact meaning depends on the wider context.

So if the scene involves a castle, basement, or hidden door, pasadizo could very naturally be understood as secret passage.

Yes, it is natural and correct.

For Spain, this sentence sounds fine:

• El pasadizo está bajo la trampilla.

A very common alternative would also be:

• El pasadizo está debajo de la trampilla.

So the sentence is good Spanish, and it fits Spain Spanish well.