El coche está en buen estado, aunque es antiguo.

Because Spanish uses estar and ser for different kinds of description.

• estar is used for a state or condition
• ser is used for a more essential or permanent characteristic

So:

• está en buen estado = it is in good condition
  → this is the car’s current condition
• es antiguo = it is old
  → this is seen as a characteristic of the car

A very common learner mistake would be El coche es en buen estado, which is not correct.

En buen estado is a very common expression meaning in good condition.

It is made up of:

• en = in
• buen = good
• estado = condition, state

Spanish often uses this kind of structure where English uses an adjective:

• Está en buen estado = It is in good condition
• Está en mal estado = It is in bad condition

This is a fixed, natural phrase, especially for cars, houses, furniture, electronics, and other second-hand items.

Because bueno becomes buen before a masculine singular noun.

So:

• un coche bueno = a good car
• un buen coche = a good car
• buen estado = good condition

This shortening happens when bueno comes directly before the noun:

• bueno → buen before masculine singular nouns
• malo → mal in the same kind of position

Since estado is masculine singular, buen estado is correct.

Because estado is a masculine noun in Spanish. Its article is el:

• el estado

So any adjective agreeing with it must also be masculine singular:

• buen estado
• mal estado

This has nothing to do with the gender of coche. In the phrase en buen estado, the adjective buen is describing estado, not coche.

Aunque means although / even though and introduces a contrast.

Here the contrast is:

• the car is in good condition
• but it is old

So the idea is: both things are true at the same time.

Aunque is a little more formal or smoother than always using pero, though both can often express contrast. In this sentence, aunque sounds very natural.

Because the speaker is presenting es antiguo as a real, known fact.

After aunque, Spanish can use either:

• indicative when the information is presented as true or accepted
• subjunctive when it is hypothetical, uncertain, or not being asserted as a fact

Here, the speaker clearly means the car really is old:

• aunque es antiguo = although it is old

Compare:

• Aunque es antiguo, funciona bien.
  The car really is old.
• Aunque sea antiguo, lo compraré.
  Even if it is old, I will buy it.

Because antiguo here describes an inherent characteristic: the car’s age.

Spanish normally uses ser for that:

• El coche es antiguo.

Using estar with antiguo would sound wrong or very unnatural in this context. A car’s age is not treated as a temporary condition.

So the contrast in the sentence is very typical:

• está en buen estado = its condition is good
• es antiguo = its age is old

It can mean different things depending on context.

With something like coche, antiguo usually means:

• old
• sometimes vintage or classic, depending on context

With objects like furniture or art, it can sometimes mean:

• antique
• ancient
• old

So coche antiguo usually means an old car, but in the right context it may suggest an older model or classic car.

Also note an important nuance:

• un amigo antiguo is not the normal way to say an old friend
• un antiguo amigo usually means a former friend

So adjective position can change the meaning.

Because aunque es antiguo is a subordinate clause adding contrast to the main statement.

In writing, Spanish often uses a comma before aunque when the aunque clause comes after the main clause:

• El coche está en buen estado, aunque es antiguo.

This is very natural punctuation.

If the aunque clause comes first, the comma normally separates the two parts too:

• Aunque es antiguo, el coche está en buen estado.

Yes, you could say:

• El coche está en buen estado, pero es antiguo.

That is also correct and natural.

The difference is mainly one of style and structure:

• pero = but
• aunque = although / even though

Aunque often sounds a bit more connected and slightly more formal or polished, but both are common.

In Spanish, articles like el, la, un, and una are used more often than in English.

So when talking about a specific car, Spanish normally says:

• El coche...

Dropping the article here would sound incomplete in a normal full sentence.

Spanish is not copying English word for word. Where English says The car is..., Spanish also naturally uses the article.

Yes, coche is the most common everyday word for car in Spain.

In much of Latin America, people more often say:

• carro
• auto
• automóvil in more formal contexts

So this sentence sounds very natural for Spain.

Yes. Both are correct.

• Está en buen estado = It is in good condition
• Está en buenas condiciones = It is in good condition / in good shape

The version with buen estado is especially common and compact.
The version with buenas condiciones is also very natural.

For a car ad or description in Spain, está en buen estado is extremely common.

Yes. Spanish allows some flexibility in word order.

For example:

• Aunque es antiguo, el coche está en buen estado.

This means the same thing, but the emphasis changes slightly. Starting with aunque es antiguo highlights the contrast first.

The original sentence is probably the most neutral and natural order:

• El coche está en buen estado, aunque es antiguo.