"くうこうは大きいです。"

Translation: Airports are big.

[JohnPMChappell]

空港は大きいです。

[Cicang]

I wrote "It is a big airport". And it was wrong. Wkwk

[V2Blast]

That would use a different sentence structure: おおきいくうこうです。 (Possibly preceded by それは.)

[Azrael89]

Why is "an airport is big" marked wrong?

[DegenerateMedia]

I assume because a plural is implied but I don't exactly know where or how, so I would appreciate if anyone could fill us in!

[Azrael89]

I think that's not the case here. Duolingo said the correct answer is "the airport is big". Although, before I also had this sentence and I answered with "airports are big" which is marked correct.
Anyway, without the context it could mean An airport is, Airports are, The airport is or The airports are.

[Eyal_Elron]

How should I know if it's an airport in singular or airports in plural?

[trishka9]

Context. You wouldn't know from this isolated sentence.

[ElijahWrig414612]

Yes. Yes they are.