Prietenii cooperează bine.

In Romanian, the definite article is usually attached to the end of the noun instead of being a separate word like the in English.

So: prieten = friend
prieteni = friends
prietenii = the friends

That means prietenii cooperează bine is most naturally understood as the friends cooperate well.

Because two different endings come together:

• prieten = friend
• prieteni = friends
• prietenii = the friends

The first i belongs to the plural form prieteni.
The second i is part of the attached definite article.

So the double i is not random spelling; it shows plural + definite.

cooperează is the present-tense form of the verb a coopera, meaning to cooperate.

In this sentence, it means cooperate. With prietenii as the subject, the full sense is the friends cooperate.

A useful detail: in Romanian, the 3rd person singular and 3rd person plural often look the same in the present tense. So:

• el cooperează = he cooperates
• ea cooperează = she cooperates
• ei cooperează = they cooperate

The subject tells you whether it is singular or plural.

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

English usually says They cooperate well, but Romanian can simply say Cooperează bine if the context already makes the subject clear.

In your sentence, the subject is already stated as prietenii, so adding ei would usually be unnecessary:

Prietenii cooperează bine. = The friends cooperate well.

You could say Ei cooperează bine, but only if they is already understood from context.

bine is an adverb, and here it means well.

It modifies the verb cooperează, so it tells us how the friends cooperate:

• cooperează = cooperate
• cooperează bine = cooperate well

This is very similar to English structure.

Prietenii cooperează bine is the most neutral, straightforward order.

Romanian word order is fairly flexible, but different orders can sound more marked or give emphasis. For example:

• Prietenii cooperează bine. = neutral
• Bine cooperează prietenii. = more emphatic or stylistically marked, something like The friends do cooperate well

For normal everyday use, the original order is the best one to learn first.

The letter ă is a very common Romanian vowel. It is pronounced as a short central sound, somewhat like the a in English sofa, though not exactly the same.

So the end of cooperează is roughly like zuh.

This sound is important because ă is a separate Romanian letter, not just a decorated a.

Yes. a coopera is a close equivalent of English to cooperate.

Because it is a cognate, it is easy for English speakers to recognize. In many contexts, Romanian can also use a colabora, which is closer to to collaborate, but a coopera is perfectly normal here.

So Prietenii cooperează bine is a very natural sentence meaning that the friends work together well.