Prietenii se antrenează pe o distanță lungă.

Because prietenii is the definite plural form: the friends.

A quick breakdown:

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

The two i letters are there for a reason:

• one i marks the plural
• the other i is the definite article attached to the end of the noun

Romanian often puts the at the end of the noun instead of using a separate word.

Se is a reflexive pronoun, and here it is part of the verb a se antrena = to train / to work out / to practice.

So se antrenează means train, but literally it has the idea of train themselves.

In natural English, we usually do not translate se word-for-word. We just say:

• Prietenii se antrenează = The friends are training

Yes.

• a antrena = to train someone, to coach, sometimes to involve
• a se antrena = to train oneself, to work out, to practice

For example:

• Antrenorul antrenează sportivii. = The coach trains the athletes.
• Sportivii se antrenează. = The athletes are training.

So in your sentence, the reflexive form is used because the friends themselves are doing the training.

Because in the present tense, many Romanian verbs have the same form in the 3rd person singular and plural.

For a se antrena:

• el/ea se antrenează = he/she trains
• ei/ele se antrenează = they train

So the verb form alone does not show singular vs. plural here. The subject tells you:

• prietenul = singular
• prietenii = plural

Here, pe is a preposition meaning something like over, across, or for a certain distance.

So:

• pe o distanță lungă = over a long distance

This is not the same pe that marks a direct object in sentences like Îl văd pe Andrei.

In your sentence, pe introduces the idea of distance.

Because distanță is a feminine singular noun.

That affects both the article and the adjective:

• o = feminine singular a
• lungă = feminine singular form of long

So:

• o distanță lungă = a long distance

Everything agrees with distanță.

Because in Romanian, adjectives usually come after the noun.

So the normal order is:

• distanță lungă = long distance
• casă mare = big house
• om bun = good man

You can sometimes put the adjective before the noun, but that often sounds more literary, emphatic, or stylistically marked.

So o lungă distanță is possible, but o distanță lungă is the more neutral everyday order.

Not completely. Romanian word order is fairly flexible, but the given sentence is the neutral, standard order:

• Prietenii se antrenează pe o distanță lungă.

You can change the order for emphasis, but it may sound more marked. For example:

• Pe o distanță lungă se antrenează prietenii.

This puts more focus on pe o distanță lungă.

One important point: in simple tenses, se normally stays before the conjugated verb:

• se antrenează, not antrenează se

Ă is a separate Romanian vowel. It sounds like the a in English about or sofa when unstressed: a short, neutral uh sound.

So:

• antrenează ends with something like -ea-zuh
• lungă ends with -guh

It is not the same as the a in father.

Yes, very often.

There is a slight difference in feel:

• pe o distanță lungă = over one long distance
• pe distanțe lungi = over long distances in general

So if you are speaking generally about long-distance training, pe distanțe lungi can sound very natural. The version in your sentence is still correct; it just sounds more specific.