Learn Hardcore Korean: yogeumi bissamyeon taeksi daesin jihacheoreul tayo. - If the fare is expensive, I take the subway instead of a taxi.

-면 is a conditional ending meaning “if/when.”
So 요금이 비싸면 means “If the fare is expensive / When the fare is expensive.” In everyday speech, this often implies “in that case.”

Why is it 요금이 and not 요금을?

Because 요금(=fare/fee) is the subject of the clause 요금이 비싸면 (“If the fare is expensive”).

요금이 = “the fare” (subject) is expensive
If you used 요금을, that would mark 요금 as an object, which doesn’t fit with the adjective 비싸다 (expensive), since adjectives describe a subject.

What’s the difference between 요금 and 가격?

요금 = a fee/fare/rate for a service (taxi fare, admission fee, phone bill rate, etc.)
가격 = the price of a product/item (the price of shoes, a laptop, etc.)
A taxi charge is naturally called 요금.

Why does Korean say 택시 대신 지하철을 타요 instead of something like “take the subway rather than a taxi”?