Avionul pleacă azi.

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

• avion = plane / a plane
• avionul = the plane

So -ul is the masculine/neuter singular definite article here.

Romanian normally uses an attached definite article instead of a separate word.

In English:

• the plane

In Romanian:

• avionul

This is one of the big differences from English. The article is often added directly to the noun.

Pleacă is the 3rd person singular present tense of a pleca, which means to leave, to depart, or sometimes to go away.

So avionul pleacă means:

• the plane leaves
• the plane is departing

With transport, a pleca is very commonly used for departures.

Romanian often leaves out subject pronouns when they are not needed.

So instead of saying:

• El pleacă
• Ea pleacă

Romanian very often just says:

• Pleacă

In your sentence, the noun avionul is already the subject, so there is no need for any extra pronoun.

Just like English, Romanian can use the present tense for a scheduled future event.

So Avionul pleacă azi can mean:

• The plane leaves today
• The plane is leaving today

This is very natural when talking about timetables, plans, or scheduled departures.

Azi means today.

Yes, azi and astăzi both mean today. In many situations they are interchangeable.

• azi = shorter, very common in everyday speech
• astăzi = also common, sometimes a little more formal or fuller-sounding

So Avionul pleacă azi and Avionul pleacă astăzi both work.

The letter ă is a very common Romanian vowel. It is a short, neutral sound, similar to the a in English about or sofa.

So pleacă is pronounced roughly like:

• PLEA-kuh

Not exactly English, of course, but that approximation helps.

A rough pronunciation is:

ah-vee-oh-NOOL PLEA-kuh AH-zee

A few notes:

• avionul → stress on the last syllable: -nul
• pleacă → stress on plea-
• azi → stress on the second syllable

This is only an approximation, but it is a useful starting point.

Yes. Romanian word order is fairly flexible, although Avionul pleacă azi is a normal, neutral order.

You might also hear:

• Azi pleacă avionul. = Today the plane leaves.
• Avionul azi pleacă. = possible, but more marked and less neutral

Changing the order usually changes emphasis, not the basic meaning.

Avion is a neuter noun in Romanian.

Romanian neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular you get:

• un avion = a plane
• avionul = the plane

That is why the singular definite article here looks like the masculine/neuter ending -ul.