כשהוא עייף, הוא לא רוצה לנגן בפסנתר; הוא רק שומע מוזיקה בשקט.

It is כש־ + הוא.

• כש־ means when
• הוא means he

So כשהוא literally means when he.

In everyday Hebrew, כש־ is very common. A more formal alternative is כאשר.

In Hebrew, the verb to be is usually not expressed in the present tense.

So:

• הוא עייף = he is tired
• literally, it is closer to he tired

That is completely normal Hebrew.

But in the past or future, Hebrew does use forms of to be:

• כשהוא היה עייף = when he was tired
• כשהוא יהיה עייף = when he is/will be tired

עייף is an adjective, and it has to agree with the subject.

Here the subject is הוא = he, so the adjective is masculine singular:

• הוא עייף = he is tired

Other forms would be:

• היא עייפה = she is tired
• הם עייפים = they are tired
• הן עייפות = they are tired

Because Hebrew normally states the subject again in a new clause:

• כשהוא עייף, הוא לא רוצה...

This is especially natural in the present tense, because forms like רוצה do not clearly mark person by themselves. רוצה can mean:

• I want
• you want (masculine singular)
• he wants

So the pronoun helps make the subject clear.

In casual speech, a pronoun can sometimes be omitted if the context is obvious, but here repeating הוא is the most natural standard wording.

In Hebrew, לא usually negates the main finite verb:

• הוא רוצה = he wants
• הוא לא רוצה = he does not want

Then the infinitive comes after it:

• הוא לא רוצה לנגן = he does not want to play

So Hebrew does not usually put the negation directly on the infinitive in a sentence like this.

This is a very common Hebrew pattern:

• רוצה + infinitive = wants to + verb

So:

• רוצה = wants
• לנגן = to play

Together:

• רוצה לנגן = wants to play

The ל־ on לנגן is the normal marker of the infinitive, like to in English.

With musical instruments, Hebrew commonly uses the preposition ב־ after לנגן.

So:

• לנגן בפסנתר = to play the piano
• לנגן בגיטרה = to play guitar
• לנגן בכינור = to play violin

This is just the normal Hebrew pattern.

By contrast, if the thing being played is a song or a piece, Hebrew can use a direct object:

• לנגן את השיר = to play the song

Because Hebrew often uses the bare noun for instruments in a general sense.

So לנגן בפסנתר means to play piano / to play the piano as a general activity.

If you want to make it more specific, you can add something like:

• בפסנתר הזה = on this piano

So the sentence is talking about the activity of piano-playing in general, not a particular piano.

Because לשמוע מוזיקה is the normal everyday way to say to listen to music.

Even though שומע literally means hears, in Hebrew it is very commonly used where English says listen to music.

Compare:

• הוא שומע מוזיקה = he listens to music
• הוא מקשיב למוזיקה = he is listening attentively to the music

So מקשיב is possible, but it sounds more focused or deliberate. שומע מוזיקה is the most natural everyday phrasing here.

רק means only or just.

Its position matters. Here:

• הוא רק שומע מוזיקה = he only/just listens to music

That means listening to music is the only thing he does in this situation.

Compare:

• הוא שומע רק מוזיקה = he listens only to music
  (not podcasts, not radio, etc.)

So in your sentence, רק modifies the whole action שומע מוזיקה, not just the noun מוזיקה.

בשקט literally means in quiet.

In this sentence it means something like:

• quietly
• softly
• in a quiet way

So the idea is that he is not actively playing; he is just listening in a calm, quiet way.

Also, ב־ is attached directly to the noun:

• שקט = quiet
• בשקט = quietly / in quiet

Yes, Hebrew allows that:

• הוא לא רוצה לנגן בפסנתר כשהוא עייף.

That is grammatical.

But putting כשהוא עייף at the beginning is very natural because it sets up the situation first:

• When he is tired, ...

So the original version gives the time/condition immediately, which is often a smooth way to organize the sentence.

The semicolon is a punctuation choice, not a special Hebrew grammar rule.

It connects two closely related clauses:

• כשהוא עייף, הוא לא רוצה לנגן בפסנתר;
• הוא רק שומע מוזיקה בשקט.

A period would also be possible:

• כשהוא עייף, הוא לא רוצה לנגן בפסנתר. הוא רק שומע מוזיקה בשקט.

So the semicolon simply shows that the two parts are closely connected.

You would change the words that agree with the subject:

• כשהיא עייפה, היא לא רוצה לנגן בפסנתר; היא רק שומעת מוזיקה בשקט.

Changes:

• הוא → היא
• עייף → עייפה
• רוצה stays רוצה in writing, but now it is understood as feminine from the subject
• שומע → שומעת

This is a good example of how Hebrew marks gender throughout the sentence.