المفتاح عند باب البيت.

A common Egyptian pronunciation is:

el-meftāḥ ʿand bāb el-bēt

A few useful notes:

• المفتاح is often pronounced el-meftāḥ in Egyptian.
• عند is ʿand. The ʿ stands for ع (ʿayn), a deep throat sound that English does not have.
• البيت in Egyptian is usually el-bēt, not al-bayt as in careful Standard Arabic.

So if you are learning Egyptian Arabic, el-meftāḥ ʿand bāb el-bēt is a good model.

Because Arabic normally leaves out the present-tense verb to be.

So in Arabic, a sentence like:

المفتاح عند باب البيت

literally looks like:

the key at the door of the house

But it means:

The key is at/by the house door.

This is completely normal in Arabic.
If you wanted past tense, then Arabic would use a verb, for example:

كان المفتاح عند باب البيت
The key was at the house door.

Here, عند means something like:

• at
• by
• near

So it gives the location of the key.

In this sentence, عند باب البيت means the key is located by/at the door of the house.

A useful comparison:

• في = in / inside
• عند = at / by / near

So:

• المفتاح في البيت = the key is in the house
• المفتاح عند باب البيت = the key is by/at the house door

Because this is an idafa construction, often called a noun-noun possessive construction.

باب البيت means:

• the door of the house
• or more naturally, the house door

In an idafa:

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

So:

• باب بيت = a house door / the door of a house
• باب البيت = the house door / the door of the house

That is why الباب البيت is not correct here.

Because المفتاح means the key, not just a key.

The prefix الـ is the Arabic definite article, like English the.

So:

• مفتاح = a key
• المفتاح = the key

Since the sentence is talking about a specific key, المفتاح is definite.

It works very well as a sentence that an Egyptian would understand, and the words are also shared with Standard Arabic.

The main difference is mostly in pronunciation:

• Standard Arabic: al-miftāḥu ʿinda bābi l-bayti
• Egyptian Arabic: el-meftāḥ ʿand bāb el-bēt

So the structure and vocabulary are fine for Egyptian learners, but the way you say it in everyday Egyptian is different from formal Standard Arabic.

This is another important feature of the idafa construction.

In باب البيت:

• باب does not have ال
• but the whole phrase is still definite because البيت is definite

So although باب looks indefinite on its own, the full phrase باب البيت means the door of the house, which is definite.

This is very common in Arabic:

• كتاب الولد = the boy's book
• باب البيت = the house door

Yes, Arabic word order can be flexible, but المفتاح عند باب البيت is the most natural neutral order here.

This sentence is a nominal sentence:

• المفتاح = topic
• عند باب البيت = comment/information about it

So it is like saying:

As for the key, it is by the house door.

You may sometimes hear other orders for emphasis, but for a learner, المفتاح عند باب البيت is the best basic pattern to use.

Yes. In normal Arabic writing, short vowel marks are usually not written.

So you see:

المفتاح عند باب البيت

But in fully vowelled Standard Arabic, it would be something like:

المفتاحُ عندَ بابِ البيتِ

In Egyptian Arabic, those case endings are not used in everyday speech, so learners of Egyptian usually focus on the spoken form:

el-meftāḥ ʿand bāb el-bēt

Yes, and that can be confusing for learners.

عند can be used for location, as in this sentence:

• المفتاح عند باب البيت = the key is by the house door

But it can also be used to express possession:

• عندي مفتاح = I have a key
• literally: at me [there is] a key

So عند does not always mean exactly the same thing in English.
Its basic idea is often something like at or with, and the exact English translation depends on context.