Le copropriétaire appelle le syndic.

Because French normally uses an article before both the subject noun and the object noun.

• le copropriétaire = the co-owner
• le syndic = the syndic / property manager / building administrator

So the sentence structure is:

• Le copropriétaire = subject
• appelle = verb
• le syndic = direct object

In English, that is simply The co-owner calls the syndic.

Appeler is the infinitive, meaning to call.
In this sentence, the verb is conjugated because it matches the subject le copropriétaire.

The subject is third person singular, so:

• j'appelle = I call
• tu appelles = you call
• il / elle / on appelle = he / she / one calls

So Le copropriétaire appelle means The co-owner calls.

Because appeler is a stem-changing verb in the present tense. In several forms, the l is doubled:

• j'appelle
• tu appelles
• il/elle/on appelle
• ils/elles appellent

But not in:

• nous appelons
• vous appelez

So appelle is the correct present-tense form here.

Copropriétaire literally means co-owner.

It is often used in the context of an apartment building or condominium, where someone owns one unit and also shares ownership of the common parts of the building.

So it is a specific legal/property word, not just a casual word for any owner.

In this context, syndic is a property-management term. It usually refers to the person or company that manages a co-owned building, especially in condo/apartment-building contexts.

Depending on the situation, English translations might include:

• building manager
• property manager
• management company
• administrator
• trustee in some legal contexts

So this is a specialized word, and it does not always have one perfect English equivalent.

Because appeler takes a direct object in French.

You call someone, so French says:

• appeler quelqu'un = to call someone

That is why the sentence uses:

• le syndic

and not:

• au syndic

If you used au, it would no longer be the normal structure for appeler.

In this sentence, le copropriétaire is masculine because of the article le.

The noun itself is often the same in both genders:

• le copropriétaire = the male co-owner
• la copropriétaire = the female co-owner

So the article tells you the gender here.

Yes. This is completely normal.

French often uses a definite article where English also uses the:

• Le copropriétaire appelle le syndic.

There is nothing unusual about starting a sentence with Le. It simply means The.

A helpful approximate pronunciation is:

co-pro-pri-é-tair

More closely in IPA, it is roughly:

[kɔpʁɔpʁijetɛʁ]

A few points:

• the r is the French r
• é sounds like ay in say, but shorter and tenser
• the final -aire sounds like air with a French r

It is a long word, so it helps to break it into parts: co | pro | prié | taire

It is pronounced approximately:

san-deek

More closely in IPA:

[sɛ̃dik]

Important points:

• syn- has a nasal vowel, so it is not pronounced exactly like English sin
• the final c is pronounced here: -dik

So do not pronounce it like silent-final-consonant words such as blanc.

Yes: it is a standard French word order:

subject + verb + object

• Le copropriétaire = subject
• appelle = verb
• le syndic = object

That is the normal order for a basic statement in French, just like in English:

The co-owner calls the syndic.

Yes, but it changes the meaning.

• Le copropriétaire appelle le syndic = The co-owner calls the syndic
  This suggests a specific co-owner already known in the context.

• Un copropriétaire appelle le syndic = A co-owner calls the syndic
  This introduces the co-owner as unspecified or not yet identified.

So the grammar is the same, but the article changes the nuance.

No, not in this exact sentence.

French changes le or la to l' only before a vowel sound or mute h:

• le syndic stays le syndic because syndic begins with a consonant sound
• le copropriétaire stays le copropriétaire for the same reason

So there is no elision here.

The grammar is completely normal, but the vocabulary is somewhat specialized.

• appelle is a very common everyday verb
• copropriétaire and syndic are more specific property/legal terms

So the sentence is natural, but it belongs especially to the context of buildings, apartments, condos, and co-ownership management.