Questa chiave appartiene a Marta.

Because chiave is a feminine singular noun in Italian, and the demonstrative has to agree with it.

• questo = this (masculine singular)
• questa = this (feminine singular)

So:

• questo libro = this book
• questa chiave = this key

Italian nouns have grammatical gender, and the ending does not always tell you everything with complete certainty.

Chiave is simply a feminine noun:

• la chiave = the key

Many feminine nouns do end in -a, but some feminine nouns end in -e too. You usually learn the gender together with the noun, often by learning the article as well:

• la chiave
• il libro
• la notte
• il pane

So it’s best to memorize chiave as la chiave.

Appartiene means belongs or belongs to.

It comes from the verb appartenere = to belong.

So:

• Questa chiave appartiene a Marta. = This key belongs to Marta.

Here, appartiene is the third-person singular form, because the subject is questa chiave = this key.

Because the subject is questa chiave, which is third-person singular.

The verb appartenere is conjugated like this in the present tense:

• io appartengo = I belong
• tu appartieni = you belong
• lui/lei appartiene = he/she belongs
• noi apparteniamo = we belong
• voi appartenete = you all belong
• loro appartengono = they belong

Since this key is a singular thing, Italian uses the same verb form as he/she/it:

• questa chiave appartiene

Because the verb appartenere is normally followed by a when you say who something belongs to.

So the pattern is:

• appartenere a qualcuno = to belong to someone

Examples:

• Il libro appartiene a Luca. = The book belongs to Luca.
• La borsa appartiene a mia sorella. = The bag belongs to my sister.

So a Marta means to Marta.

Yes. That is also correct and very common.

• Questa chiave appartiene a Marta.
• Questa chiave è di Marta.

Both mean This key belongs to Marta / This is Marta’s key.

A small difference in feel:

• appartiene a sounds a little more explicit or formal
• è di is often more everyday and very natural in speech

Because in standard Italian, personal names usually do not take an article.

So you say:

• Marta
• Luca
• Giulia

Not normally:

• la Marta
• il Luca

In some regional varieties or informal speech, especially in parts of Italy, articles before names can appear, but for a learner, the normal standard rule is:

• a Marta = to Marta

The normal word order is:

• Questa chiave appartiene a Marta.

That is the most neutral and natural order.

Italian can sometimes change word order for emphasis, but this standard order is what you should use first:

1. subject = Questa chiave
2. verb = appartiene
3. complement = a Marta

So for learners, this is the safest pattern to copy.

Yes.

• Questa chiave appartiene a Marta. = This key belongs to Marta.
• La chiave appartiene a Marta. = The key belongs to Marta.

The difference is the same as in English:

• questa points to a specific key as this key
• la chiave means the key

So questa is not required by the verb; it just adds the idea of this.

Chiave is pronounced roughly like KYAH-veh.

A few helpful points:

• chi before a/o/u gives a hard k sound
• so chiave begins with k, not like English ch in chair
• the ia is part of the same syllable area, giving kya-

So:

• chiave ≈ KYAH-veh

Also:

• chiesa = church, pronounced roughly KYEH-zah
• chiuso = closed, roughly KYOO-zoh

Mostly yes, but the structure is important.

In English:

• This key belongs to Marta.

In Italian:

• Questa chiave appartiene a Marta.

So the Italian pattern is:

• [thing] + appartenere + a + person

That part matches English pretty well. The main thing to remember is the preposition a, not di after appartenere.

Correct:

• appartiene a Marta

Not:

• appartiene di Marta

Yes. It can mean to belong to in a broader sense too.

For example:

• Marta appartiene a un club sportivo. = Marta belongs to a sports club.
• Questo stile appartiene al passato. = This style belongs to the past.

So it can refer to:

• ownership
• membership
• category/classification

In your sentence, though, it clearly means ownership: the key belongs to Marta.