El jugador no s'entrena sol.

It stands for es, the 3rd-person pronominal/reflexive pronoun.

Before a vowel, es is shortened to s', so:

• es entrena → s'entrena

The dictionary form is entrenar-se.

A few related forms:

• m'entreno = I train
• t'entrenes = you train
• s'entrena = he/she trains

Because Catalan very often uses the pronominal form entrenar-se when talking about a person training, working out, or practicing.

It is related to the idea of training oneself, but in normal English we usually just say train rather than train oneself.

Compare:

• entrenar algú / un equip = to train someone / a team
• entrenar-se = to train, to work out

So in this sentence, s'entrena is the natural form for saying that the player is doing the training himself.

That is the normal way to make a negative sentence in Catalan.

The basic pattern is:

• subject + no + verb

And if there is a clitic pronoun, no still goes before the verb group:

• El jugador no s'entrena sol.

This is different from English, which uses do/does not. Catalan does not need an extra helping verb here.

Here, sol means alone or by himself.

It is not the adverb només meaning only.

So:

• sol = alone
• només = only

Also, this sol is an adjective agreeing with the subject, even though in English alone does not change form.

Because sol agrees with jugador, which is masculine singular.

Agreement works like this:

• sol = masculine singular
• sola = feminine singular
• sols = masculine plural
• soles = feminine plural

So:

• El jugador no s'entrena sol.
• La jugadora no s'entrena sola.
• Els jugadors no s'entrenen sols.
• Les jugadores no s'entrenen soles.

Because that is the most natural, neutral position here.

In this sentence, sol describes the subject after the verb, much like English alone in He doesn’t train alone.

Putting it at the end sounds normal and unmarked:

• El jugador no s'entrena sol.

If you move it, you usually create emphasis or a different nuance.

Because Catalan normally uses an article with a common noun when referring to a specific person or thing.

So:

• el jugador = the player
• un jugador = a player

Using el here tells you we are talking about a specific player, not just any player.

Yes, if the context already makes it clear who you are talking about.

Catalan often leaves out the subject when it is understood, especially with verbs that already show the person and number.

So you could say:

• No s'entrena sol.

That would still be a complete sentence if the listener already knows which person is being discussed. Including El jugador simply makes the subject explicit.