الباب المقاوم للحريق مغلق.

It is a nominal sentence in Modern Standard Arabic.

You can divide it like this:

• الباب المقاوم للحريق = the subject/topic, meaning the fire-resistant door
• مغلق = the predicate, meaning closed

So the structure is:

[the fire-resistant door] [closed]

which in natural English becomes The fire-resistant door is closed.

In Arabic, the verb to be is usually not written or said in the present tense in sentences like this.

So:

• الباب مغلق = The door is closed
• not literally The door closed, but that is how Arabic expresses is closed in the present

If you want the past or future, Arabic does use a verb such as:

• كان الباب مغلقًا = The door was closed
• سيكون الباب مغلقًا = The door will be closed

Because in Arabic, adjectives normally come after the noun they describe.

So:

• الباب المقاوم = the resistant door
• then للحريق completes that idea: resistant to fire

This is the normal Arabic order:

• noun first
• adjective after it

Unlike English, where we usually say fire-resistant door.

Because Arabic adjectives usually match the noun they describe in definiteness.

Here:

• الباب = the door
• so the adjective must also be definite:
• المقاوم = the resistant

That is why you get:

• الباب المقاوم = the resistant door

If the noun were indefinite, the adjective would also be indefinite:

• بابٌ مقاومٌ = a resistant door

This matching is very important in Arabic.

Because مغلق is not part of the noun phrase the fire-resistant door. It is the predicate of the whole sentence.

So the sentence is:

• الباب المقاوم للحريق = the fire-resistant door
• مغلق = is closed

In other words:

• المقاوم is an adjective inside the subject
• مغلق is the predicate of the sentence

This difference is very important.

Compare:

• الباب المقاوم للحريق مغلق = The fire-resistant door is closed
• الباب المقاوم للحريق المغلق = the closed fire-resistant door

In the second one, المغلق has الـ and becomes just another adjective, not the predicate.

A very useful clue is definiteness.

• الباب is definite because it has الـ
• المقاوم is also definite, so it matches the noun and acts as an adjective
• مغلق is indefinite, so it works naturally as the predicate

So Arabic is signaling the structure for you:

• الباب المقاوم للحريق = one complete noun phrase
• مغلق = what is being said about that noun phrase

للحريق is a prepositional phrase attached to المقاوم.

It means something like:

• resistant to fire
• literally, the لـ here helps express what the door resists

So:

• المقاوم للحريق = resistant to fire / fire-resistant

This is a common pattern in Arabic:

• مقاوم للماء = water-resistant
• مقاوم للصدأ = rust-resistant
• مقاوم للحريق = fire-resistant

So the لـ after مقاوم is very natural and common.

No, not between الباب and المقاوم.

It is not an iḍāfa because:

• in an iḍāfa, the first noun normally does not take الـ
• here both الباب and المقاوم can be definite

So this is better understood as:

• الباب = noun
• المقاوم = adjective modifying it
• للحريق = prepositional phrase modifying the adjective

So the whole phrase is an adjective phrase, not an iḍāfa.

A fully vowelled version would be:

البابُ المُقاوِمُ لِلحَريقِ مُغلَقٌ

Here is what the endings show:

• البابُ: nominative, because it is the subject/topic
• المُقاوِمُ: nominative too, because it is an adjective matching الباب
• لِلحَريقِ: genitive after the preposition لِ
• مُغلَقٌ: nominative, because it is the predicate

In normal Arabic writing, these short vowels are usually omitted, so you usually see:

الباب المقاوم للحريق مغلق

Without vowel marks, مغلق could in theory be read more than one way. But in this sentence, the intended reading is clearly مُغلَق = closed.

Why?

1. Context: a door is naturally described as closed
2. Meaning of the sentence: it is stating the condition of the door
3. The common passive/adjectival reading here is مُغلَق

So the intended form is:

• مُغلَق = closed

not:

• مُغلِق = closing / one who closes

Arabic often relies on context when short vowels are not written.

Both words relate to fire, but they are not used in exactly the same way.

• نار is the everyday word for fire
• حريق usually means a fire, blaze, or fire incident, and it is very common in safety and technical language

That is why expressions like fire-resistant often use:

• مقاوم للحريق

This sounds more like the standard safety/technical expression in Modern Standard Arabic.