ليش الباب مفتوح؟

ليش means why. It is very common in Levantine Arabic and everyday spoken Arabic more generally.

A few useful points:

• In formal / Standard Arabic, you would usually use لماذا instead.
• In everyday Levantine speech, ليش is the normal choice.
• You may also hear it pronounced a little differently depending on the region, but ليش is the standard Levantine form learners usually start with.

In Arabic, the verb to be is usually not expressed in the present tense.

So:

• الباب مفتوح literally looks like the door open
• but it means the door is open

That is completely normal Arabic grammar.
If you wanted a past or future meaning, then Arabic would use a verb:

• كان الباب مفتوح = the door was open
• رح يكون الباب مفتوح = the door will be open

So in ليش الباب مفتوح؟, the meaning is naturally Why is the door open?

Arabic often forms this kind of question by simply putting why in front of a normal statement.

Statement:

• الباب مفتوح = The door is open

Question:

• ليش الباب مفتوح؟ = Why is the door open?

So the structure is basically:

• ليش
  • the door is open

English needs is before the subject in this kind of question, but Arabic does not. Arabic keeps the basic statement structure and adds the question word.

باب means door.
الباب means the door.

The prefix الـ is the Arabic definite article, equivalent to the in English.

So:

• باب = a door / door
• الباب = the door

In this word, the l sound of الـ is pronounced clearly because ب is a moon letter, not a sun letter. So it is pronounced roughly il-baab or el-baab in Levantine, depending on the speaker.

Here, مفتوح is functioning as an adjective, meaning open.

So:

• الباب مفتوح = The door is open

Historically, مفتوح is related to the verb فتح (to open) and has the pattern of a passive participle, so it literally has the sense of opened. But in everyday usage, in a sentence like this, you should think of it simply as open.

Compare:

• الباب مفتوح = The door is open
• الباب مسكّر = The door is closed

Yes. Adjectives in Arabic normally agree with the noun they describe in:

• gender
• number
• definiteness

Here:

• الباب is masculine singular
• so مفتوح is also masculine singular

If the noun changed, the adjective would also change. For example:

• الشباك مفتوح = The window is open
• البابين مفتوحين = The two doors are open
• الأبواب مفتوحة = The doors are open
• الشباك مفتوحة would be wrong if شباك is being treated as masculine

A very common thing for learners to notice is that the adjective after a definite noun does not need الـ here, because in predicate sentences like this, it stays indefinite:

• الباب مفتوح = The door is open not
• الباب المفتوح = the open door

That last one is a different structure.

This is an important difference.

• الباب مفتوح = The door is open
  This is a full sentence.
• الباب المفتوح = the open door
  This is a noun phrase, not a complete sentence by itself.

So in your sentence:

• ليش الباب مفتوح؟ = Why is the door open?

But:

• ليش الباب المفتوح...؟ would be the beginning of something like Why the open door...?, which is incomplete unless more words follow.

A common Levantine pronunciation is roughly:

leesh il-baab maftooH?

A few pronunciation notes:

• ليش = leesh
• الباب = il-baab or el-baab
• مفتوح = maftooH

The final ح in مفتوح is important. It is a breathy, deeper h sound made in the throat, not the ordinary English h.

If you say it naturally, the stress often sounds something like:

• LEESH il-BAAB maf-TOOH?

Yes, but it sounds much more formal or written.

In everyday Levantine conversation, people normally say:

• ليش الباب مفتوح؟

Using لماذا is grammatically understandable, but it usually sounds like:

• formal Arabic
• news language
• writing
• school-style language

So if your goal is natural spoken Levantine, ليش is the better choice.

It can be either, depending on tone and context.

It may simply be a neutral question:

• Why is the door open?

But it can also sound like:

• Why did you leave the door open?
• What’s going on?
• Shouldn’t the door be closed?

That kind of slightly accusatory meaning often comes from:

• intonation
• facial expression
• situation

So the words themselves are neutral, but in real conversation they can easily carry emotion.

A few simple answers would be:

• لأنو في هوا = Because there’s air / a draft
• لأنو حدا فات = Because someone came in
• نسيتو مفتوح = I forgot it open / I forgot to close it
• عم نهوي الغرفة = We’re airing out the room
• ما بعرف = I don’t know

A very common pattern for answering ليش questions is:

• لأنو... = because...

It is definitely Levantine-style spoken Arabic, but many Arabic speakers from other regions would still understand it.

Why?

• ليش is widely used or at least widely recognizable in spoken Arabic
• الباب and مفتوح are very common words

That said, another dialect might prefer a different everyday word for why, or pronounce parts differently. So it is natural Levantine, but not so regional that others would fail to understand it.