باب البيت مقفول بالليل.

This is because Arabic is using an idafa structure, often called a possession construction.

• باب = door
• البيت = the house

When two nouns are put together like this:

• باب البيت

it means:

• door of the house
• or more naturally in English, the house door / the door of the house

In an idafa:

• the first noun usually does not take ال-
• the second noun shows whether the whole phrase is definite

So because البيت is definite (the house), the whole phrase باب البيت becomes definite: the door of the house.

In Arabic, especially in the present tense, you usually do not say a separate word for is / am / are.

So:

• باب البيت مقفول literally looks like:
• the house door locked

but it means:

• The door of the house is locked

This is a normal nominal sentence in Arabic. The verb to be is understood in the present.

If you wanted the past, then Arabic would use a word like كان:

• باب البيت كان مقفول = The house door was locked

مقفول means locked or sometimes closed/shut, depending on context.

In this sentence, because we are talking about a door, مقفول most naturally means:

• locked

It comes from the root related to closing/locking. In Egyptian Arabic, مقفول is very common in everyday speech.

You may also hear it used for other things, for example:

• المحل مقفول = The shop is closed
• التليفون مقفول = The phone is off / unavailable
  (literally closed)

So the exact English translation depends on the situation.

In this sentence, مقفول is functioning like an adjective or descriptive state: locked.

So:

• باب البيت = the subject
• مقفول = what is being said about it

This is similar to saying:

• The door is locked

It is not acting like a full verb here. It is more like a predicate adjective in English.

In Egyptian Arabic, بالليل is a very common way to say:

• at night
• during the night

The بـ here often works like in / at / during, especially in time expressions.

So:

• بالليل = at night

You may also see في الليل, especially in more formal Arabic or in Standard Arabic, but in everyday Egyptian speech بالليل is very natural.

It breaks down like this:

• بـ = in / at / during
• الليل = the night

So:

• بالليل literally = in the night
• natural English = at night

This kind of time phrase is very common in Egyptian Arabic.

In Egyptian Arabic, البيت is usually pronounced more like:

• il-bēt

rather than the Standard Arabic-style al-bayt.

A rough pronunciation of the whole phrase is:

• bāb il-bēt maʔfūl bil-lēl

Notes:

• الـ is often pronounced il- in Egyptian
• بيت sounds like bēt
• بالليل sounds like bil-lēl

In many varieties of Egyptian Arabic, ق is pronounced as a glottal stop ء, not as a strong q sound.

So مقفول is commonly pronounced:

• maʔfūl

not:

• maqfūl

This glottal stop is the small catch in the throat that English speakers sometimes make in the middle of uh-oh.

So the word sounds roughly like:

• ma'fool, with a brief stop before the f

Because باب is a masculine singular noun.

In Arabic, adjectives usually agree with the noun they describe in gender and number.

• باب = masculine singular
• so: مقفول = masculine singular

If the noun were feminine, you would usually use مقفولة.

For example:

• الشنطة مقفولة = The bag is closed
• الباب مقفول = The door is locked

Yes, you could, if the context already makes it clear which door you mean.

Compare:

• باب البيت مقفول بالليل = The house door is locked at night
• الباب مقفول بالليل = The door is locked at night

The version with باب البيت is more specific: it tells you it is the house’s door.

The version with الباب is simpler and more natural if everyone already knows which door you are talking about.

The order is:

• باب البيت = subject/topic
• مقفول = predicate
• بالليل = time expression

So the structure is basically:

• [The door of the house] [locked] [at night]

Arabic often puts the main thing being talked about first, and then gives information about it.

So this is a very normal Egyptian Arabic sentence pattern.

By itself, بالليل usually means at night / during the night in a general sense.

So:

• باب البيت مقفول بالليل

usually suggests a general habit or normal situation:

• The house door is locked at night

It does not necessarily mean only one specific night. It often sounds like a routine or usual fact unless the context says otherwise.

Sometimes in context, yes, but not automatically.

Literally, باب البيت means:

• the door of the house

In many real-life situations, English would naturally interpret that as:

• the house door
• possibly the front door

But Arabic itself is not specifically saying front door. If you wanted to be more specific, you could say something more exact depending on context.

So the phrase is general unless the situation makes it clear which door is meant.

A common natural pronunciation would be:

• bāb il-bēt maʔfūl bil-lēl

Word by word:

• باب → bāb
• البيت → il-bēt
• مقفول → maʔfūl
• بالليل → bil-lēl

That is a good practical pronunciation model for everyday Egyptian Arabic.