Learn Hardcore Japanese: hikouki ha hayai desu ga, takai desu.

は is the topic marker (sometimes called the “contrastive” or “theme” particle). Here it tells us that “as for airplanes…” we’re going to say something about them. It doesn’t necessarily mark the doer of an action (that’s が in many cases); it just sets up what we’re talking about.

What is the role of が in this sentence?

In this context, が is a conjunction meaning “but”. It connects two clauses:

“飛行機は速いです” (Airplanes are fast)
“高いです” (They are expensive)
So it works like the English word “but.”

Why are there two です endings? Can they be omitted?

Each です attaches to an adjective to make it polite:

速い → 速いです
高い → 高いです
In casual speech you can drop です:
飛行機は速いが高い。 (less formal)
Often you’d also swap が for けど in casual:
飛行機は速いけど高い。

Why isn’t there a second は or が before 高いです?

Once you’ve established the topic (“飛行機は…”), you don’t need to repeat it. The second clause implicitly refers back to the same topic. Omitting the particle makes the sentence smoother.

Can I use other words besides が to say “but”?

Yes. Common alternatives:

けど (casual)
けれど(も) (a bit more formal than けど)
しかし (very formal or written style)
Each has its own nuance and formality level.

Is there a difference between the kanji 速い and 早い?

Yes. Although both read はやい, they mean different things:

速い refers to speed or velocity.
早い refers to earliness or soonness in time.
Here we use 速い because we’re talking about how fast airplanes travel.

Could I swap the order: “高いですが、飛行機は速いです”?

You technically can, but it changes the nuance. Starting with 高いですが highlights “they’re expensive, but…” and then you add the positive point “they’re fast.” The original order (“fast but expensive”) puts the positive feature first.

What’s the difference between using adjectives directly and using ～いです?

Adjectives in Japanese come in two main types. い-adjectives like 速い already contain an “-い” ending, so you can say:

Plain: 速い
Polite: 速いです
You don’t need any extra verb. In contrast, な-adjectives require な before a noun or です after: e.g. きれい → きれいです. Here both 速い and 高い are い-adjectives, so just tack on です for politeness.