معاينة الشقة اليوم، ومدة العقد مهمة.

Because Arabic often uses nominal sentences in the present tense, with no word for is/are.

This sentence is best read as two clauses:

• معاينة الشقة اليوم = The apartment viewing is today
• ومدة العقد مهمة = And the contract term is important

So Arabic does not need a separate present-tense verb here.
If you wanted the past or future, you would usually add a verb such as كانت or ستكون.

معاينة is a verbal noun (called a maṣdar) from the verb عاين, which means to inspect, to examine, or to view.

So معاينة can mean things like:

• viewing
• inspection
• examining

In this sentence, it works like a noun, not a verb. That is why it can be the subject of a nominal sentence.

Because معاينة الشقة is a noun phrase meaning the viewing of the apartment or viewing the apartment.

This is an iḍāfa-like structure with a verbal noun:

• معاينة = viewing / inspection
• الشقة = the apartment

Arabic often links nouns directly like this, where English uses of or an object after a verbal noun.

So:

• معاينة الشقة = viewing the apartment
• literally: inspection/viewing of the apartment

In fully vowelled Arabic, الشقة would be in the genitive here: مُعايَنَةُ الشَّقَّةِ

اليوم means today, and here it gives the time.

In معاينة الشقة اليوم, it functions as an adverbial expression of time and serves as the predicate of the nominal sentence.

So the idea is:

• معاينة الشقة = the apartment viewing
• اليوم = today

Together: The apartment viewing is today.

مدة العقد is an iḍāfa construction, meaning the duration of the contract or the contract term.

It breaks down like this:

• مدة = duration / length / term
• العقد = the contract

Arabic does not need a separate word for of here.
So مدة العقد literally means duration the-contract, but naturally in English it becomes the duration of the contract.

Because it agrees with مدة, which is a feminine singular noun.

In the second clause:

• مدة العقد = subject
• مهمة = predicate adjective, meaning important

Arabic adjectives and predicate adjectives agree with the noun in gender and number, so:

• مدة is feminine singular
• therefore مهمة is feminine singular

The comma matters here: مهمة belongs to مدة العقد, not to both parts of the sentence together.

Yes. The comma strongly suggests that this is two separate statements, not one long coordinated subject.

With the comma, the reading is:

• معاينة الشقة اليوم = The apartment viewing is today
• ومدة العقد مهمة = And the contract term is important

Without the comma, a learner might try to read it as one combined subject, something like Viewing the apartment today and the contract term..., which would create agreement questions later in the sentence.

So the comma helps show the intended structure.

A fully vowelled version would be:

مُعايَنَةُ الشَّقَّةِ اليَوْمَ، وَمُدَّةُ العَقْدِ مُهِمَّةٌ.

Here is the grammar behind those endings:

• مُعايَنَةُ: nominative, because it is the subject of the first nominal sentence
• الشَّقَّةِ: genitive, because it is the second term in the iḍāfa-like structure
• اليَوْمَ: accusative, as an adverbial expression of time
• مُدَّةُ: nominative, because it is the subject of the second nominal sentence
• العَقْدِ: genitive, because it is the second term of the iḍāfa
• مُهِمَّةٌ: nominative, because it is the predicate of the second nominal sentence

Yes, Arabic word order is somewhat flexible, but changing it can affect emphasis.

For example:

• معاينة الشقة اليوم = neutral: The apartment viewing is today
• اليوم معاينة الشقة = more emphasis on today

Both can work, but the original order is very natural if the topic is the apartment viewing.