Părerea lui se schimbă.

Because părerea means the opinion. Romanian often expresses possession as the X of someone, so his opinion is literally the opinion of him:

• părere = opinion
• părerea = the opinion

So părerea lui = his opinion.

Also, părere is a feminine noun, and its definite singular form is made by adding the article to the end:

• părere → părerea

Lui means of him or his.

In Romanian, possession is very often expressed with the possessed thing first and the owner after it:

• părerea lui = literally the opinion of him
• natural English translation: his opinion

So lui is not the subject here. It just shows whose opinion it is.

Yes. Părerea sa can also mean his opinion.

The difference is mostly one of style and clarity:

• părerea lui = very common in everyday Romanian, clearly his
• părerea sa = also correct, often a bit more formal or literary

One important point: sa can also mean her, depending on context. Lui is clearer if you specifically mean a male person.

Because the verb here is a se schimba, which means to change or to become different.

Compare:

• a schimba = to change something
  • El schimbă planul. = He changes the plan.
• a se schimba = to change / to undergo change
  • Părerea lui se schimbă. = His opinion changes / is changing.

So se is part of the verb here. It does not literally mean the opinion is doing something to itself in an English-style reflexive sense. It is just how Romanian forms this intransitive idea.

It is present tense, third person singular.

Depending on context, it can mean:

• changes
• is changing

So Părerea lui se schimbă can translate as either:

• His opinion changes
• His opinion is changing

Romanian present tense often covers both the simple present and the present progressive.

Because the subject is părerea = the opinion, which is singular.

Even though lui refers to a person, lui is not the subject. The subject is the noun părerea.

So:

• Părerea lui se schimbă. = singular subject
• Părerile lui se schimbă. = plural subject, his opinions change

A useful detail: in the present tense, the third-person singular and plural of many Romanian verbs look the same, including se schimbă.

Yes, Romanian word order is more flexible than English word order.

The neutral version is:

• Părerea lui se schimbă.

But other orders are possible for emphasis or style, for example:

• Se schimbă părerea lui.
• Lui i se schimbă părerea.

These do not radically change the core meaning, but they shift emphasis. For a learner, Părerea lui se schimbă is the safest and most natural basic order.

No. Another common word is opinie.

So you can also say:

• Opinia lui se schimbă.

Both are correct. Very roughly:

• părere often feels more everyday and personal
• opinie can sound a bit more formal or abstract

But in many contexts, they are close in meaning.

You add nu before the verb:

• Părerea lui nu se schimbă.

This means:

• His opinion does not change
• His opinion is not changing

That is the normal Romanian way to negate a verb.

This sentence simply says that his opinion is changing. It does not focus on who causes the change.

With a se schimba, Romanian expresses the idea that something undergoes change:

• Părerea lui se schimbă. = His opinion is changing.

If you want to say that someone actively changes it, Romanian would usually use the non-reflexive verb:

• Cineva îi schimbă părerea. = Someone is changing his opinion / mind.

So the contrast is:

• se schimbă = changes / is changing
• schimbă ceva = changes something

The letter ă is pronounced like a short, unstressed uh sound, similar to the vowel in English sofa or about.

Very rough approximations:

• părerea ≈ puh-RE-rea
• schimbă ≈ SKIM-buh

These are only approximate, but they are helpful for beginners. Romanian ă is one of the most common sounds in the language, so it is worth getting used to early.