من الضروري أن يبلغ المستأجر مالك العقار بالعطل.

In Arabic, the present-tense verb to be is usually omitted.

So من الضروري is a common impersonal expression meaning it is necessary.
You can think of it as a set phrase, similar to:

• من المهم = it is important
• من الأفضل = it is better

So the sentence does not need a separate word for is.

أن introduces a following verb phrase and is often translated as to or that, depending on the English wording.

Here it links it is necessary with the action:

• من الضروري أن... = it is necessary that... / it is necessary to...

It also makes the following imperfect verb subjunctive, which affects its ending.

In this sentence, it should be read as يُبَلِّغَ.

That is from بلّغ = to inform / notify.

This is important because, without vowels and the shadda, يبلغ could be misread. In normal unvowelled Arabic writing, short vowels and doubling marks are usually omitted, so learners have to identify the correct reading from context.

Here the meaning makes it clear that it is:

• يُبَلِّغَ = that he notify / inform

not a form meaning reach.

Because Arabic very often uses verb–subject–object order, especially in formal writing.

So inside the clause after أن, the structure is:

• يُبَلِّغَ = verb
• المستأجرُ = subject
• مالكَ العقارِ = object

This is a very normal Arabic word order.

المستأجر means the tenant or the renter.

It comes from the verb استأجر = to rent / hire.

In this sentence, المستأجر is the person doing the action of notifying, so it is the grammatical subject.

Because this is an iḍāfa construction, often called a possessive or noun-link construction.

• مالك = owner
• العقار = the property / real estate

Together:

• مالك العقار = owner of the property

In an iḍāfa, the first noun usually does not take ال if the second noun already makes the whole phrase definite.

So:

• مالك العقار = the property owner

not المالك العقار.

Because the verb بلّغ takes the person informed as a direct object.

So Arabic says, literally:

• notify the property owner

not:

• notify to the property owner

That is why مالك العقار comes directly after the verb with no preposition.

بالعطل means about the fault / defect / malfunction.

Here بـ introduces the thing being reported. With بلّغ, Arabic can use بـ for the information or issue someone is being notified about.

So the structure is:

• يُبَلِّغَ مالكَ العقارِ بالعطلِ
• notify the property owner about the defect

The noun العطل here refers to a problem, breakdown, or malfunction.

In fully vocalized form, it would be approximately:

مِنَ الضَّروريِّ أَنْ يُبَلِّغَ المُستأجِرُ مالِكَ العَقارِ بِالعُطلِ

The important case roles are:

• المستأجرُ = nominative, because it is the subject
• مالكَ = accusative, because it is the direct object
• العقارِ = genitive, because it is the second term of an iḍāfa
• العطلِ = genitive, because it comes after بـ

Also, after أن, the verb is subjunctive:

• يُبَلِّغَ

In normal Arabic writing, these endings are usually not written.

Literally, it means owner of the property.

In context, though, it often works just like landlord, especially when the sentence is about a tenant reporting a problem.

So:

• literal sense: the owner of the property
• natural English in this context: the landlord

Both ideas fit, but property owner is the more literal wording.