Suo quan m'entreno.

Suo is the 1st person singular present tense of suar, meaning to sweat.

So:

• suo = I sweat

Catalan often leaves out the subject pronoun, so suo already includes the idea of I.

Because Catalan is a language that often drops subject pronouns when they are not needed.

The ending of the verb already tells you who the subject is:

• suo = I sweat
• sues = you sweat
• sua = he/she/it sweats

So Suo quan m'entreno is perfectly natural. You could say Jo suo quan m'entreno, but that usually adds emphasis or contrast.

Here quan means when and introduces a time clause.

So the structure is:

• Suo = main clause
• quan m'entreno = subordinate time clause

It means something like I sweat when I work out / train.

M' is the shortened form of em, a weak pronoun meaning myself / me here.

Before a vowel, em becomes m':

• em rento → m'rento? No, that would not happen because rento starts with a consonant, so it stays em rento
• em entreno → m'entreno

So m'entreno comes from em + entreno.

Because the verb is being used reflexively: entrenar-se = to train / work out oneself.

So:

• m'entreno = I train / I work out
• entreno by itself can also exist, but it may sound less clearly reflexive and can also mean I train/coach someone or something, depending on context.

For a learner, m'entreno is the clearest way to express I work out.

Most naturally, it sounds habitual or general:

• Suo quan m'entreno = I sweat when I work out

That is, whenever I work out, I sweat.

Catalan present tense often covers this kind of general truth or regular situation, just like English present simple.

Yes. That is also correct.

• Suo quan m'entreno
• Quan m'entreno, suo

Both mean the same thing. The difference is mostly focus:

• Suo quan m'entreno starts with the main idea: I sweat
• Quan m'entreno, suo starts with the time setting: When I work out

Also, if the quan clause comes first, a comma is normally used:

• Quan m'entreno, suo.

But in the original order, you normally do not use a comma:

• Suo quan m'entreno.

A simple learner-friendly guide would be:

• suo ≈ SU-o
• quan ≈ kwan
• m'entreno ≈ men-TRE-no

A couple of useful notes:

• suo is usually pronounced with two syllables
• the apostrophe in m'entreno does not create a pause
• the stress in entreno falls on tre

So the rhythm is roughly:

SU-o kwan men-TRE-no

Yes, that is grammatical.

But jo is usually unnecessary unless you want to stress I:

• Jo suo quan m'entreno, però ell no.
  = I sweat when I work out, but he doesn’t.

Without that kind of contrast, the more natural version is usually just:

• Suo quan m'entreno.