المسرح مفتوح النهارده.

A common pronunciation is:

el-masraḥ maftūḥ en-naharda

A few quick notes:

• المسرح → el-masraḥ
• مفتوح → maftūḥ
• النهارده → en-naharda

You may also see transliterations like il-masraḥ maftūḥ ennaharda.
The ḥ stands for the Arabic letter ح, a stronger, breathier h sound than English h.

It is Egyptian Arabic.

The biggest clue is النهارده, which is the Egyptian colloquial word for today. In Modern Standard Arabic, you would more commonly say:

المسرح مفتوح اليوم

So the sentence structure is very straightforward, but the vocabulary is clearly Egyptian.

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

So Arabic often says, literally:

the theater open today

but it means:

the theater is open today

This is completely normal.

If you wanted a different time, then Arabic would use a word for was/will be, for example:

• المسرح كان مفتوح امبارح = The theater was open yesterday.
• المسرح هيبقى مفتوح بكرة = The theater will be open tomorrow.

Because here مفتوح is a predicate adjective, not an adjective inside a noun phrase.

This sentence means:

The theater is open today.

So the structure is:

• المسرح = the theater
• مفتوح = open

In Arabic, predicate adjectives are usually indefinite, so you do not put ال on مفتوح here.

Compare:

• المسرح مفتوح = The theater is open.
• المسرح المفتوح = the open theater

That second one is not a full sentence by itself; it is just a noun phrase.

Because it matches المسرح, which is grammatically masculine singular.

In Arabic, predicate adjectives agree with the noun in gender and number. So:

• masculine singular: مفتوح
• feminine singular: مفتوحة

For example:

• الصيدلية مفتوحة = The pharmacy is open.

Since المسرح is masculine singular, مفتوح is the correct form.

In Egyptian Arabic, النهارده means today.

It is a very common everyday word in speech and informal writing.

A useful detail: even though it comes from a word related to daytime, the full expression النهارده means today, not during the day.

If you want to say during the day, you would usually say:

بالنهار

So:

• النهارده = today
• بالنهار = in the daytime / during the day

This is because of the sun letter / moon letter rule.

• In المسرح, the first letter after ال is م, which is a moon letter, so the l sound is pronounced:
  el-masraḥ

• In النهارده, the first letter after ال is ن, which is a sun letter, so the l sound assimilates to the next sound:
  en-naharda, not el-naharda

So the spelling stays with ال, but the pronunciation changes depending on the next letter.

Yes, you can.

Both are natural, but the emphasis changes a little:

• المسرح مفتوح النهارده = neutral statement about the theater
• النهارده المسرح مفتوح = emphasizes today

In English, the second one feels a bit like:

Today, the theater is open.

So the original sentence is the more neutral, unmarked order, but fronting النهارده is completely possible.

The most straightforward Egyptian Arabic negative is:

المسرح مش مفتوح النهارده
= The theater is not open today.

Another very common way is to use closed instead:

المسرح مقفول النهارده
= The theater is closed today.

So:

• مش مفتوح = not open
• مقفول = closed

Both are useful and common.

Yes, sometimes.

In Egyptian Arabic:

• مفتوح describes the state open
• فاتح is also commonly used for a place being open for business / operating

So you may hear things like:

• المحل فاتح = The shop is open.
• البنك فاتح النهارده = The bank is open today.

With المسرح, مفتوح sounds very natural.
فاتح could also be understood, but مفتوح is a very safe and standard choice here.

مسرح can mean theater, but depending on context it can also refer to a stage or a theatrical venue more generally.

In this sentence, the intended meaning is clearly the place itself, as in a theater being open.

So a learner can safely understand المسرح here as the theater.