Learn Hardcore Japanese: kuruma ha takai desu.

は is the topic marker. It tells the listener “as for the car…” and frames the rest of the sentence. Japanese often introduces what you’re talking about first (the topic) and then gives information about it.

Could we use が instead of は in this sentence?

Yes, but the nuance changes. が marks the grammatical subject and often introduces new information or emphasizes the subject.

車が高いです → “It’s the car that is expensive (not something else).”
車は高いです → “Speaking of the car, it is expensive.”

Why do we add です after 高い? Can we drop it?

です is the polite copula.

In casual speech you can drop です: 車は高い.
In polite speech you include です: 車は高いです.
Without です, the sentence sounds more informal.

What kind of word is 高い, and how do i-adjectives work?

高い is an i-adjective (ends with い). I-adjectives inflect directly for tense and polarity:

Negative: 高くない (not expensive)
Past: 高かった (was expensive)
Polite past: 高かったです

Why aren’t there articles like “a” or “the” before 車?

Japanese has no direct equivalent of English articles. Context tells you whether it’s “the car,” “a car,” or just “cars in general.” If you need to be specific, you add a demonstrative (e.g. その車 “that car”).

How do I make this sentence negative or past tense politely?

Polite negative: 車は高くないです or more formally 車は高くありません.
Polite past: 車は高かったです.
Combine inflected i-adjective forms with です for politeness.

What is the word order in 車は高いです?

Japanese generally uses Topic–Comment order:

Topic (車は)
Predicate (Adjective + Copula: 高いです)
So it’s Topic → Adjective → Copula, which corresponds loosely to “The car is expensive.”

Can we specify “that car” instead of just “car” here?

Yes. Use a demonstrative:

その車は高いです → “That car is expensive.”
この車は高いです → “This car is expensive.”

How would I ask “Is the car expensive?” using this structure?

Simply add the question particle か at the end:
車は高いですか。
Literally “As for the car, is (it) expensive?”