Avantajul se schimbă repede.

Because -ul is the Romanian definite article attached to the end of the noun.

• avantaj = an advantage / advantage
• avantajul = the advantage

Romanian usually does not use a separate word for the the way English does. Instead, the article is added to the noun itself.

Avantaj is a neuter noun.

In Romanian, neuter nouns behave:

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

So you get:

• singular: un avantaj, avantajul
• plural: două avantaje, avantajele

That is why the singular definite form is avantajul.

Here, se is a reflexive/pronominal marker used with the verb a se schimba.

In this sentence, se schimbă means something like:

• changes
• is changing
• becomes different

English often does not translate se directly. It is just part of how this verb is commonly used in Romanian.

A very useful contrast is:

• a schimba = to change something
• a se schimba = to change, to change oneself, to become different

So in Avantajul se schimbă repede, the idea is that the advantage changes.

Usually no, not if you want the meaning the advantage changes quickly.

Without se, schimbă is normally transitive, so it expects an object:

• Avantajul schimbă situația. = The advantage changes the situation.

But in your sentence, the subject itself is what changes, so Romanian normally uses se schimbă.

So:

• Avantajul se schimbă repede. = correct for The advantage changes quickly
• Avantajul schimbă repede. = sounds incomplete, because it feels like something is missing after schimbă

Because the subject is avantajul, which is third person singular.

The verb is in the present tense, third person singular:

• eu mă schimb
• tu te schimbi
• el/ea se schimbă

Since avantajul = it / the advantage, Romanian uses se schimbă.

It can mean both.

Romanian present tense often covers both:

• the English simple present
• the English present continuous

So depending on context, Avantajul se schimbă repede could mean:

• The advantage changes quickly
• The advantage is changing quickly

Usually the surrounding context tells you which one sounds more natural in English.

Because Romanian does not need it here.

The subject is already stated explicitly as avantajul, so there is no need to add a pronoun.

Romanian also often omits subject pronouns even when there is no noun, because the verb ending already shows the person clearly.

So Romanian naturally says:

• Avantajul se schimbă repede.

not something like:

• Avantajul el se schimbă repede.

Adding el here would sound unnecessary or emphatic.

Avantajul se schimbă repede is the most neutral and natural order.

In Romanian, adverbs like repede often come after the verb phrase. So this placement is very normal.

Word order in Romanian is somewhat flexible, but different orders can sound marked or unnatural without special context.

For example:

• Avantajul se schimbă repede. = neutral, standard
• Se schimbă repede avantajul. = possible, but marked; it would need a specific context
• Avantajul repede se schimbă. = much less natural in ordinary speech

Also notice that se normally stays right before the conjugated verb:

• se schimbă

A rough English-style pronunciation is:

ah-vahn-TAH-zhool seh SKIM-buh REH-peh-deh

A few useful details:

• j in avantajul sounds like the s in measure
• sch in schimbă sounds like sk, not English sh
• ă sounds like the first vowel in about
• r in Romanian is usually a light tapped or rolled r

So:

• avantajul ≈ ah-vahn-TAH-zhool
• schimbă ≈ SKIM-buh
• repede ≈ REH-peh-deh

You add nu before the verb phrase:

Avantajul nu se schimbă repede.

This means The advantage does not change quickly or The advantage isn’t changing quickly.

Notice the order:

• nu se schimbă

The reflexive marker se stays with the verb, and nu comes before it.