Tramvaj je prazniji u lipnju nego u rujnu.

Prazniji is the comparative form of prazan.

• prazan = empty
• prazniji = emptier / less full

Croatian often forms comparatives with endings like -iji, -ji, or -ši. Here the adjective uses the stem prazn-, so you get prazniji.

It also agrees with tramvaj, which is masculine singular, so the form is prazniji. If the noun were feminine singular, you would get praznija.

Je is the 3rd person singular present of biti (to be), so it means is.

So:

• Tramvaj je prazniji = The tram is emptier

Croatian normally uses je in this kind of sentence, where the subject is linked to an adjective.

Because after u in this sentence, Croatian uses the locative form of the month name.

So:

• lipanj → u lipnju
• rujan → u rujnu

English keeps the basic form (in June, in September), but Croatian changes the noun form depending on case.

Because each time expression needs its own preposition.

So Croatian says:

• u lipnju nego u rujnu

not:

• u lipnju nego rujnu

The word nego means than, but it does not replace the preposition u. You still need u before rujnu.

Usually, in this sentence, nego is the natural choice.

After a comparative, Croatian can compare things in two common ways:

• nego
  • phrase/clause
• od
  • genitive noun phrase

For example:

• Ivan je viši nego Marko.
• Ivan je viši od Marka.

But in your sentence, the comparison is between two time phrases:

• u lipnju
• u rujnu

So nego u rujnu sounds natural and standard.

Yes. Prazan literally means empty, but with places and vehicles it can also mean not crowded or less full.

So with tramvaj, prazniji can mean:

• emptier
• less crowded
• not as full

Depending on context, English might translate it a little differently, but the Croatian is perfectly natural.

Because in Croatian, month names are normally written with lowercase letters.

So you write:

• u lipnju
• u rujnu

not:

• u Lipnju
• u Rujnu

This is different from English, where June and September are capitalized.

Croatian does not have articles like a, an, or the.

So tramvaj can mean:

• a tram
• the tram

Which one is meant depends on the context. In an English translation, the tram is often the most natural choice here, but Croatian does not need a separate word for that.

Yes. Croatian word order is fairly flexible.

These are all possible:

• Tramvaj je prazniji u lipnju nego u rujnu.
• Tramvaj je u lipnju prazniji nego u rujnu.
• U lipnju je tramvaj prazniji nego u rujnu.

The basic meaning stays the same, but the focus changes slightly.

• The original version sounds fairly neutral.
• Starting with U lipnju puts more emphasis on the time.

A few useful pronunciation points:

• j is pronounced like English y in yes
• nj is pronounced like ny in canyon

So:

• tramvaj has a y-like sound at the end
• prazniji contains j = y
• lipnju ends with nj = ny
• rujnu has j = y

Croatian spelling is quite consistent, so once you learn these letter-sounds, pronunciation becomes much easier.