مفتاح احتياطي عند مالك العقار.

In Arabic, present-tense sentences often omit to be.

So مفتاح احتياطي عند مالك العقار is understood as something like:

• A spare key is with the property owner
• or, more literally, A spare key [is] at/with the property owner

This is normal in Modern Standard Arabic.

عند usually means at, by, or with.
When it is used with a person, it often gives the idea of being in someone’s possession.

So عند مالك العقار means:

• with the property owner
• in the property owner’s possession
• held by the property owner

It is not just physical location; it can also imply possession or availability.

Because Arabic adjectives normally come after the noun they describe.

So:

• مفتاح = key
• احتياطي = spare / backup

Together:

• مفتاح احتياطي = a spare key

Also, the adjective has to match the noun in things like gender, number, and definiteness.

Because the phrase is indefinite: a spare key, not the spare key.

So:

• مفتاح احتياطي = a spare key
• المفتاح الاحتياطي = the spare key

In Arabic, the adjective matches the noun in definiteness too, so if the noun has ال, the adjective must also have ال.

Yes, it can.

Although beginners often learn that the topic of a nominal sentence is usually definite, indefinite beginnings are also possible in several situations. One common case is when the noun is described, as here:

• مفتاح احتياطي = not just a key, but a spare key

This makes it more acceptable and natural. In concise notices or practical statements, this kind of wording is very common.

English often uses there is in similar situations, but Arabic does not need to.

This is an iḍāfa construction, often called a possessive or genitive construction.

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

So:

• مالك العقار = owner of the property

Arabic usually does not use a separate word for of here. It just puts the two nouns together.

This is a very common pattern:

• باب البيت = the door of the house
• مدير المدرسة = the principal of the school
• مالك العقار = the owner of the property

Because in an iḍāfa construction, the first noun normally does not take ال.

So the correct form is:

• مالك العقار

not:

• المالك العقار

The definiteness comes from the second noun. Since العقار is definite, the whole phrase مالك العقار is definite: the owner of the property.

Not always exactly.

Literally, مالك العقار means the owner of the property.

In many real-life contexts, that may be the same person as the landlord, so translating it as landlord is often fine. But strictly speaking:

• مالك العقار = property owner
• مؤجِّر = lessor / landlord

So مالك العقار focuses on ownership, not necessarily on the rental relationship.

With full case endings, it would be:

مفتاحٌ احتياطيٌّ عندَ مالكِ العقارِ

A quick breakdown:

• مفتاحٌ: nominative
• احتياطيٌّ: nominative adjective matching مفتاح
• عندَ: usually written with fatḥa
• مالكِ: genitive after عند
• العقارِ: genitive because it is the second part of the iḍāfa

In normal Arabic writing, these short vowels are usually omitted, so you just see:

مفتاح احتياطي عند مالك العقار

This is natural, especially in a short note, notice, or practical message.

But there are other possible versions depending on the context:

• مفتاح احتياطي عند مالك العقار = concise, note-like
• يوجد مفتاح احتياطي عند مالك العقار = There is a spare key with the property owner
• المفتاح الاحتياطي عند مالك العقار = The spare key is with the property owner

So the version you have is brief and natural, especially when the goal is just to give information efficiently.