المستأجر يبلغ مالك العقار بالمفتاح الاحتياطي.

Because normal Arabic writing usually leaves out short vowels and the shadda, so يبلغ can represent more than one verb form.

In this sentence, it is understood as يُبَلِّغُ = informs / notifies / conveys to.

Without vowels, the same spelling could also be read as يَبْلُغُ = reaches / attains.
Context tells you which one is meant.

So here the full reading is:

المستأجرُ يُبَلِّغُ مالكَ العقارِ بالمفتاحِ الاحتياطيِّ

It is basically:

• المستأجر = the tenant
• يبلغ = informs / notifies
• مالك العقار = the property owner
• بالمفتاح الاحتياطي = about the spare key

So the structure is:

subject + verb + object + prepositional phrase

Because مالك العقار is an iḍāfa construction, also called a construct phrase.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite

So:

• مالك عقار = an owner of a property
• مالك العقار = the owner of the property

That is why مالك has no ال, even though the whole phrase is definite.

It is an iḍāfa phrase:

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

Literally, it means owner of the property.

In natural English, that may be translated as:

• the property owner
• sometimes the landlord, depending on context

Because it is the second noun in an iḍāfa.

In Arabic, the second part of an iḍāfa is put in the genitive. So:

• مالكُ العقارِ = owner of the property
• بابُ البيتِ = door of the house
• مفتاحُ السيارةِ = key of the car

So العقار is genitive because it is linked to مالك in the meaning owner of the property.

Because this verb pattern commonly uses بـ to introduce the thing being reported or notified.

So:

• بلّغ شخصًا بشيء = inform someone about something

Here:

• مالك العقار is the person being informed
• بالمفتاح الاحتياطي is the thing he is being informed about

Also, once بـ is used, the following noun becomes genitive.

In this sentence, it means about the spare key.

That can feel strange at first, because بـ often means with, by, or using. But Arabic prepositions do not always match English prepositions word-for-word. Their meaning depends a lot on the verb they go with.

With يُبَلِّغُ here, بـ introduces the topic or item being communicated, so the meaning is about / regarding / concerning.

Because الاحتياطي is an adjective describing المفتاح, and Arabic adjectives must agree with the noun they describe.

They agree in:

• definiteness
• gender
• number
• case

Since المفتاح is:

• definite
• masculine
• singular
• genitive after بـ

the adjective must match it:

• المفتاحِ الاحتياطيِّ

So الاحتياطي also takes ال.

A fully vocalized version would be:

المستأجرُ يُبَلِّغُ مالكَ العقارِ بالمفتاحِ الاحتياطيِّ

The endings are:

• المستأجرُ — nominative, because it is the subject
• يُبَلِّغُ — verb
• مالكَ — accusative, because it is the direct object
• العقارِ — genitive, because it is the second term of the iḍāfa
• بالمفتاحِ — genitive, because it comes after بـ
• الاحتياطيِّ — genitive, because it is an adjective agreeing with المفتاح

Both word orders are possible in Modern Standard Arabic.

You can say:

• المستأجرُ يُبَلِّغُ مالكَ العقارِ بالمفتاحِ الاحتياطيِّ
• يُبَلِّغُ المستأجرُ مالكَ العقارِ بالمفتاحِ الاحتياطيِّ

Both are correct.

Starting with المستأجر makes the sentence feel a bit more like The tenant, he informs..., where the tenant is the topic. Starting with the verb is more classically verb-initial.

In English terms, the difference is usually about style or emphasis, not a big change in core meaning.

Yes, often it can.

Literally, مالك العقار means owner of the property. In many real-life contexts, that person is the landlord, so translating it that way is natural.

But the literal Arabic phrase is a bit more specific and transparent:

• مالك = owner
• العقار = property / real estate

So landlord is often a good translation, but property owner is the more literal one.