席が

a seat

→ 席
phrase Uncommon #13,275
a seat が marks 席 as the subject; it often has no direct equivalent in English.
JA: 席が空いています。
EN: A seat is available.

その駅で席が一つ空いた。

A seat became available at that station.

どこの席がいいですか?

Which seat would you like?

まだ席が空いてるじゃないか!

There are still seats available!

— タイタニック (1997)

出来るだけ安い席がいいんですけど。

I'd like the cheapest seats available, if possible.

別の席がいい、別に構わないけど。

I’d rather have a different seat, but I don’t really mind.

席が取ってある。

A seat has been reserved.

席が取ってあるからそこにある。

Your seat is reserved, so it’s over there.

席が空いたよ

A seat has opened up.

— 自転車泥棒 (1948)

運よくいい席がとれたんだ。

I was lucky enough to get a good seat.