Garde ton pion jusqu’à ce que le dé s’arrête.

Because this is a command in the tu form: Garde ton pion = Keep your piece/token.

In French, -er verbs like garder form the informal singular imperative by using the tu form without the final -s:

• tu gardes = you keep
• garde ! = keep!

So garde here is correct because the sentence is telling one person what to do informally.

Ton means your when speaking to one person informally.

So:

• ton = your (to one person, informal)
• votre = your (formal singular or plural)
• son = his/her/its

Because the command uses the informal imperative garde, the possessive naturally matches that same informal tu relationship:

• Garde ton pion = Keep your piece.

If you were speaking formally or to several people, you would say:

• Gardez votre pion.

In this context, pion means a game piece, token, or sometimes pawn.

The exact English translation depends on the game:

• in chess, pion often means pawn
• in board games, it often means piece or token

So here it is not about a school monitor or any other meaning of pion; it is clearly the little playing piece used in a game.

Jusqu’à ce que means until.

It introduces an action that continues up to the moment when something else happens:

• Garde ton pion jusqu’à ce que le dé s’arrête.
• Keep your piece until the die stops.

It helps to treat jusqu’à ce que as a fixed expression. French learners often want to translate word by word, but it is better to recognize the whole chunk:

• jusqu’à = until / up to
• ce que is part of the structure here

So the full connector is jusqu’à ce que = until.

Because jusqu’à ce que is one of the common French expressions that normally requires the subjunctive.

So after jusqu’à ce que, French uses forms like:

• jusqu’à ce qu’il vienne
• jusqu’à ce que tu finisses
• jusqu’à ce que le dé s’arrête

Here, the subjunctive form of s’arrêter for il / elle / on / le dé is s’arrête.

This does not mean the action is unreal. It is just the grammar required after this expression.

Because the verb is s’arrêter, not just arrêter.

These two verbs are related but different:

• arrêter = to stop something
• s’arrêter = to stop

Examples:

• J’arrête la musique. = I stop the music.
• La musique s’arrête. = The music stops.

In your sentence, the die is not stopping something else; it itself stops. So French uses the reflexive/pronominal verb:

• le dé s’arrête = the die stops

Yes. English is a little irregular here.

In French:

• un dé = a die
• des dés = dice

So le dé is singular: the die.

In everyday English, many people say the dice even when speaking loosely, but strictly speaking:

• die = one
• dice = more than one

French keeps this distinction very clearly with dé / dés.

Also, dé has an accent to distinguish it from de.

The apostrophe shows elision, which is very common in French.

1. jusqu’à

This comes from:

• jusque à → jusqu’à

French avoids the clash of vowels, so jusque loses its final -e before à.

2. s’arrête

This comes from:

• se arrête → s’arrête

Again, French drops the vowel before another vowel sound.

This is normal and required in standard French spelling.

Because the fixed expression is jusqu’à ce que.

French often uses set connectors that do not match English word for word. Even though English simply says until, French says jusqu’à ce que before a clause with a verb.

Compare:

• jusqu’à demain = until tomorrow
  
  • no verb follows
• jusqu’à ce que le dé s’arrête = until the die stops
  
  • a whole clause follows

So when a new clause follows, ce que is part of the standard structure.

Not with the same meaning.

• jusqu’à ce que = until
• quand = when

Your sentence means the person should continue doing something up to a certain point:

• Keep your piece until the die stops.

If you used quand, it would sound more like the stopping is simply the time at which something happens, not the limit up to which the action continues.

So jusqu’à ce que is the natural choice here.

Yes, it is informal singular because of garde and ton.

Formal singular or plural would be:

• Gardez votre pion jusqu’à ce que le dé s’arrête.

So the changes are:

• garde → gardez
• ton → votre

Everything else stays the same.

A careful pronunciation would be roughly:

gard ton pyon zhusk-a suh kuh luh day sa-ret

A few useful points:

• Garde: the final -e is not pronounced strongly
• pion sounds roughly like pyon
• jusqu’à flows together because of the apostrophe
• dé sounds like day
• s’arrête has the sound sa-ret

In natural speech, the whole sentence links together quite smoothly.