Learn Hardcore Romanian: Autobuzul este mai ieftin decât metroul.

Mai is the marker for the comparative degree in Romanian. When you put mai before an adjective, you’re saying “more + adjective” or “–er” in English. So mai ieftin literally means “more cheap,” i.e. “cheaper.”

Why do we use decât here instead of ca?

In standard Romanian, decât is the conjunction used after a comparative (mai … decât) to introduce the second element. Colloquially you might hear ca (“mai bun ca mine”), but the prescriptive, grammatically correct form is decât for inequalities.

What is the “-ul” ending on autobuzul and metroul?

Romanian marks the definite article as a suffix on the noun. For masculine singular nouns ending in a consonant or “-u,” you add -ul. Thus autobuz (a bus) becomes autobuzul (the bus), and metrou (a subway/train) becomes metroul (the subway).

Why is the adjective in the masculine form (ieftin) and not ieftină or another form?

Adjectives in Romanian agree in gender and number with the noun they describe. Here autobuzul is masculine singular, so the adjective stays in its masculine singular form ieftin. If you compared a feminine noun, say mașina, you’d say mașina este mai ieftină.

Can I omit este and just say Autobuzul mai ieftin decât metroul like in English (“Bus cheaper than subway”)?

No. Romanian requires the copula este (is) in this kind of full comparative sentence. Dropping este would make the phrase ungrammatical or at best very colloquial and incomplete.

What if I want to say “A bus is cheaper than the subway” instead of “The bus…”?

Simply replace the definite article suffix on autobuz with the indefinite article un. You get:
Un autobuz este mai ieftin decât metroul.
(The subway remains definite because you’re comparing against a specific system, “the subway.”)

Could I flip the comparison to say “Metroul este mai scump decât autobuzul”?

Absolutely. That sentence means “The subway is more expensive than the bus.” You’ll notice you switch both the adjective to mai scump (“more expensive”) and the role of the two nouns accordingly, but the structure (subiect + este + mai/adjectiv + decât + obiect) stays the same.