السوق مفتوح يوم الاربع.

A natural Egyptian pronunciation would be:

es-sūʔ maftūḥ yōm il-arbaʿ

A few useful points:

• السوق is pronounced es-sūʔ, not al-sūq
• مفتوح = maftūḥ
• يوم = yōm
• الاربع / الأربع = il-arbaʿ

You may also hear a very colloquial version with فاتح instead of مفتوح: es-sūʔ fātiḥ yōm il-arbaʿ

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

So Arabic often says:

• السوق مفتوح = literally the market open
• but it means the market is open

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

If you wanted a past meaning, then you would use a verb such as كان:

• السوق كان مفتوح = the market was open

مفتوح means open and is commonly used as an adjective/past participle.

In this sentence, it works like the predicate:

• السوق = the market
• مفتوح = open

So السوق مفتوح means the market is open.

In Egyptian speech, مفتوح is understandable and natural. But for shops, businesses, offices, etc., Egyptians also very often use فاتح:

• المحل فاتح
• السوق فاتح

So the sentence you were given is good, but فاتح is also very common in everyday Egyptian.

Because السوق is grammatically masculine.

Adjectives in Arabic normally agree with the noun they describe or refer to:

• masculine singular noun → masculine singular adjective
• feminine singular noun → feminine singular adjective

So:

• السوق مفتوح = correct
• if the noun were feminine, you would use مفتوحة

For example:

• المدرسة مفتوحة = the school is open

يوم literally means day, but in this kind of sentence it helps mean on a certain day.

So:

• يوم الاربع = on Wednesday

In Egyptian Arabic, using يوم with weekdays is very common and natural.

You can sometimes leave it out if the context is clear:

• السوق مفتوح الاربع

But السوق مفتوح يوم الاربع sounds clearer and more idiomatic for a learner.

Because this is colloquial Egyptian-style usage, not full formal Standard Arabic.

In Standard Arabic, Wednesday is:

• الأربعاء

In Egyptian everyday speech, people often shorten it to:

• الأربع
• and in casual writing you may also see الاربع

So this sentence reflects spoken Egyptian more than careful formal spelling.

A native speaker texting informally might write:

• الاربع
• الأربع

Both are common in casual contexts.

Because س is a sun letter.

The Arabic definite article is written ال, but its pronunciation changes before certain letters. With sun letters, the l sound disappears and the next consonant is doubled.

So:

• السوق is written with ال but pronounced:
• es-sūʔ

The same kind of thing happens in words like:

• الشمس → esh-shams
• السماء → es-sama

This is a pronunciation rule only. The spelling stays ال.

In Egyptian Arabic, ق is very often pronounced as a glottal stop ʔ rather than q.

So:

• Standard-style pronunciation: sūq
• Egyptian pronunciation: sūʔ

That is why السوق in Egyptian sounds like:

• es-sūʔ

This is one of the most noticeable features of Egyptian Arabic pronunciation.

This is a very common Arabic sentence pattern called a nominal sentence:

• السوق = the topic/subject
• مفتوح = the comment/predicate

So the structure is basically:

• the market + open

That gives the meaning the market is open.

If you put words together differently, you may change the meaning. For example:

• السوق المفتوح means the open market not the market is open

So the order in your sentence is important.

It is best thought of as Egyptian-friendly everyday Arabic, with some forms that overlap with Standard Arabic.

Why it feels mixed:

• السوق and مفتوح are common in both Standard Arabic and speech
• يوم الاربع is very colloquial Egyptian-style wording
• the spelling الاربع is informal rather than formal

A very formal Standard Arabic version would be closer to:

• السوق مفتوح يوم الأربعاء

A very colloquial Egyptian version might be:

• السوق فاتح يوم الأربع

So the sentence you have is perfectly useful for learning, especially if your goal is everyday Egyptian speech.