Tramvaiul este mai rapid decât autobuzul.

Because Romanian usually puts the definite article at the end of the noun, not as a separate word before it.

• tramvai = tram
• tramvaiul = the tram
• autobuz = bus
• autobuzul = the bus

The ending -ul is a very common definite article ending for masculine/neuter singular nouns.

Romanian often uses the definite singular to talk about something in a general, class-wide sense.

So:

• Tramvaiul este mai rapid decât autobuzul.

can mean something like:

• The tram is faster than the bus
• or more generally, A tram is faster than a bus

depending on context.

This is normal Romanian usage. English often uses a/an or a bare plural for general statements, but Romanian frequently prefers the definite form.

Romanian usually makes the comparative with:

• mai + adjective

So:

• rapid = fast
• mai rapid = faster

This is similar to more + adjective in English, although in translation it often corresponds to English -er.

A useful related pattern:

• rapid = fast
• mai rapid = faster
• cel mai rapid = the fastest

Because rapid is an adjective, while repede is usually an adverb.

In this sentence, tramvaiul is being described, so Romanian uses an adjective:

• Tramvaiul este mai rapid. = The tram is faster.

But if you describe how it moves, you use an adverb:

• Tramvaiul merge mai repede. = The tram goes/moves faster.

So a good shortcut is:

• after a fi / este, you often need an adjective
• with an action verb like merge, you often need an adverb

Here, decât means than.

So:

• mai rapid decât autobuzul = faster than the bus

This is the standard word used after a comparative in Romanian.

You may also hear ca instead of decât in some speech, but in standard Romanian, decât is the safer choice in sentences like this.

You can say both.

• este = full form
• e = shorter, very common spoken form

So these are both correct:

• Tramvaiul este mai rapid decât autobuzul.
• Tramvaiul e mai rapid decât autobuzul.

The first sounds a bit fuller or more formal; the second is very natural in everyday speech.

Yes. Romanian adjectives agree with the noun in gender and number.

Here, the singular form is:

• rapid

Compare:

• tramvaiul este rapid = the tram is fast
• mașina este rapidă = the car is fast
• tramvaiele sunt rapide = the trams are fast

So in your sentence, rapid matches the singular noun it describes.

Because Romanian handles definiteness differently from English.

English:

• the tram

Romanian:

• tramvaiul

Instead of putting the before the noun, Romanian usually attaches it to the noun itself. This is one of the most important structural differences between the two languages.

The normal, neutral order here is:

• Subject + este + mai + adjective + decât + noun

So:

• Tramvaiul este mai rapid decât autobuzul.

This is the most natural order for a basic statement.

Romanian can change word order for emphasis, but that usually sounds marked or stylistic. For a learner, the sentence as given is the best model to use.

A rough English-friendly guide is:

• trahm-VAI-ool ES-teh mai rah-PEED deh-KUHT ow-toh-BOO-zool

A few helpful notes:

• ai in mai and tramvaiul sounds like eye
• r is usually rolled or tapped
• ă and especially â do not have exact English equivalents
• decât has that Romanian â sound, which is central and short

You do not need perfect pronunciation right away, but it helps to notice that Romanian spelling is usually quite consistent.