Learn Hardcore Spanish (Spain): El cinturón es de cuero y combina con mi camisa. - The belt is made of leather and matches my shirt.

Es de cuero = It is made of leather (a permanent characteristic of the belt).
Está de cuero is wrong here. Estar is not used for material.
Es cuero would mean It is leather in a very general sense (as if the belt itself were just a piece of leather). For objects, people almost always say es de cuero, es de madera, es de plástico, etc.

So El cinturón es de cuero is the standard, natural way to say The belt is made of leather.

What’s the difference between cuero and piel in Spain? Could I say es de piel?

Both cuero and piel can refer to leather, but there is a nuance:

cuero: literally leather (the treated animal hide). Very common for things like belts, shoes, bags.
piel: literally skin; in clothing, often used for leather but can sound a bit more elegant or fashion‑oriented.

In Spain:

es de cuero – very normal and straightforward.
es de piel – also correct; often used in shops and fashion contexts.

For a belt, both El cinturón es de cuero and El cinturón es de piel are fine.

Why do we say El cinturón with el? In English we just say The belt or My belt, so why not Cinturón or Mi cinturón?

In Spanish, singular countable nouns almost always need an article or a determiner (like a possessive).

El cinturón = the belt (a specific belt, understood from context).
Mi cinturón = my belt.
Just Cinturón on its own would sound like a label or an exclamation, not a normal sentence subject.

Here, El cinturón is natural if you’ve already mentioned the belt or you are clearly talking about a specific belt, not just any belt.

What exactly does combina con mean here? Is it more like matches, goes with, or combines with?

In this context, combina con means that two items of clothing look good together in terms of color or style.

combinar con algo = to match something, to go with something.

So combina con mi camisa is best translated as:

It matches my shirt
or
It goes well with my shirt,

not literally It combines with my shirt.

Why do we say combina con and not just combina or combina a?

The verb combinar, when used with the meaning to match / go well together (visually), normally takes the preposition con:

Este cinturón combina con mi camisa.
Esos zapatos no combinan con ese vestido.

Using a here would be incorrect, and omitting the preposition (combina mi camisa) would change the meaning or sound wrong.

So the pattern is: combinar con + noun.

What form of the verb is combina? Why not combino or combinan?

Combina is:

Present indicative
3rd person singular
From the verb combinar

It agrees with the subject el cinturón (which is 3rd person singular):

Yo combino
Tú combinas
Él / Ella / Usted combina ← used here
Nosotros combinamos
Vosotros combináis
Ellos / Ellas / Ustedes combinan

So El cinturón combina... is grammatically required because cinturón is singular.

Could I change the word order and say El cinturón combina con mi camisa y es de cuero? Is that still correct?

Yes, that sentence is also correct and natural:

El cinturón es de cuero y combina con mi camisa.
El cinturón combina con mi camisa y es de cuero.

Both work. The difference is only in what you mention first:

Material first, then matching.
Matching first, then material.

There’s no change in meaning, just a slightly different emphasis.

Are there other common ways to say combina con mi camisa in Spain?

Yes, some common alternatives are:

Hace juego con mi camisa.
Very idiomatic; literally “makes a pair with my shirt”, but it means It matches my shirt.
Va bien con mi camisa.
Literally “goes well with my shirt.”

All of these are natural in Spain:

combina con mi camisa
hace juego con mi camisa
va bien con mi camisa

Why is it mi camisa and not la camisa? When do you use mi versus la for clothes?

In this sentence you are clearly talking about your own shirt, so mi camisa (my shirt) is the normal choice.

Spanish can sometimes use the definite article la / el instead of a possessive when the owner is obvious, especially with body parts and clothes in certain contexts:

Me quito la camisa. = I take my shirt off.
Ponte el cinturón. = Put your belt on.

But if you specifically want to highlight whose it is, or there’s any ambiguity, you use the possessive:

mi camisa = my shirt
tu camisa = your shirt
su camisa = his / her / their / your (formal) shirt

Here mi camisa makes it explicit that it is your shirt.

What’s the difference between camisa and camiseta in Spain?

In Spain:

camisa usually means a more formal shirt, often with a collar and buttons (like a dress shirt).
camiseta is a T‑shirt (casual, usually no buttons, short sleeves).

So:

mi camisa = my (buttoned) shirt.
mi camiseta = my T‑shirt.

If you say El cinturón combina con mi camiseta, listeners will picture a more casual outfit.

How is cinturón pronounced, and what does the accent mark do?

cinturón is pronounced approximately:

[thin-tu-ROHN] in most of Spain
- ci = like the th in thin (Castilian c before i/e).
[sin-tu-ROHN] in much of Latin America
- ci = like see.

The written accent on ó tells you the stress is on the last syllable: cin-tu-RÓN.
Without the accent (cinturon), the default stress rules would make the pronunciation unclear or incorrect.

How do I say the same idea in the plural, like The belts are leather and go with my shirts?

You need to make the nouns, verbs, and possessives agree in number:

Los cinturones son de cuero y combinan con mis camisas.

Changes:

El cinturón → Los cinturones (singular → plural).
es → son (3rd person singular → plural of ser).
combina → combinan (3rd person plural of combinar).
mi camisa → mis camisas (plural possessive + plural noun).