دي اول مرة اروح البحر بالليل.

دي is the Egyptian Arabic feminine form of this.

In this sentence, دي أول مرة... literally means this is the first time...
So دي is pointing to مرة because مرة is a feminine noun.

Because مرة is feminine.

In Egyptian Arabic:

• ده = this for masculine nouns
• دي = this for feminine nouns

Since the sentence is built around أول مرة (the first time), the speaker uses دي to match مرة.

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

So:

• دي أول مرة literally looks like this first time
• but it means this is the first time

This is completely normal in both Egyptian Arabic and Standard Arabic.

أول مرة means the first time.

It is a very common expression in Arabic for talking about a first experience.

Examples:

• دي أول مرة أجي هنا = This is the first time I come here / I've come here
• دي أول مرة أشوفه = This is the first time I see him / I've seen him

So in your sentence, دي أول مرة sets up the idea of a new experience.

This is a very common thing in Arabic.

After expressions like أول مرة (the first time), Egyptian Arabic often uses the imperfect/present form:

• أروح = I go
• but in context it means I have gone / I’m going as part of this is the first time I’ve gone

So دي أول مرة أروح البحر بالليل is naturally understood as:

• This is the first time I’ve gone to the beach at night
• or This is the first time I’m going to the beach at night

The exact English translation depends on context, but the Arabic structure is normal.

In Egyptian Arabic, verbs of motion often take the destination directly, without a separate word for to.

So:

• أروح البحر = I go to the sea / beach
• أروح البيت = I go home
• أروح الشغل = I go to work

This is very natural in Egyptian Arabic.
English needs to, but Egyptian Arabic often does not.

Literally, البحر means the sea.

But in Egyptian Arabic, أروح البحر very often means go to the beach or go to the seaside, not just physically go into the water.

So depending on context, it may sound more natural in English as:

• go to the sea
• go to the beach
• go to the seaside

بالليل means at night or during the night.

It is made of:

• بـ = at / in
• الليل = the night

So literally it is something like in the night, but the natural English meaning is at night.

A very common opposite is:

• بالنهار = during the day / in the daytime

A natural Egyptian pronunciation would be:

di awwil marra arūḥ il-baḥr bil-lēl

A few pronunciation notes:

• دي = di
• أول sounds like awwil
• مرة = marra
• أروح = arūḥ
• البحر = il-baḥr
• بالليل = bil-lēl

The ḥ in أروح and البحر is a strong Arabic ح, not the normal English h.

Yes, but the structure is a little different.

• دي أول مرة أروح البحر بالليل = This is the first time I’ve gone to the beach at night
• بروح البحر بالليل لأول مرة = I’m going to the beach at night for the first time

So:

• دي أول مرة... = this is the first time...
• لأول مرة = for the first time

Both are correct, but they are used in different sentence patterns.

Because normal Arabic writing usually does not include short vowel marks.

So learners often see:

• دي اول مرة اروح البحر بالليل

instead of the fully marked version.

This is completely normal in everyday Arabic writing. Native speakers rely on context to know how the words are pronounced.

Also, in informal writing, people often write اول instead of أول, leaving out the hamza mark. That is very common in casual text.

It is clearly Egyptian Arabic.

A few clues:

• دي for this is Egyptian colloquial
• أروح in this kind of everyday structure sounds colloquial
• بالليل is very common in spoken Arabic

In Standard Arabic, you would expect a different style, such as:

• هذه أول مرة أذهب إلى البحر ليلًا

That sounds much more formal and bookish. Your sentence is natural spoken Egyptian Arabic.