El peón cambia de casilla.

In Spanish, it is very common to use the definite article with singular countable nouns when you are talking about a specific thing: el peón = the pawn.

If you said just peón, it would usually sound incomplete here. Spanish generally uses articles more often than English in simple statements like this.

Here, peón means pawn, the chess piece.

Be careful: peón can also mean a laborer or worker in other contexts. Spanish often relies on context to tell you which meaning is intended.

The accent mark shows where the stress goes: pe-ÓN.

Without the accent, peon would not follow the intended pronunciation. In Spanish spelling, accent marks are often used to show irregular stress patterns.

Cambia is the third-person singular present tense of cambiar.

So it can mean:

• he/she changes
• it changes
• the pawn changes / moves

Because the subject is el peón, the verb must be third-person singular.

Cambiar basically means to change.

In a sentence like cambiar de casilla, it means to change squares or to move to a different square. So even if the English translation uses move, the Spanish is built around the idea of changing from one square to another.

This is a very important pattern: cambiar de + noun.

It means to change from one X to another X:

• cambiar de ropa = to change clothes
• cambiar de tema = to change the subject
• cambiar de casa = to move house
• cambiar de casilla = to change square

So de does not mean only from here in a literal sense. It is part of the structure used with cambiar.

If you used a, that would suggest movement to a specific destination, but that is not the normal pattern with cambiar in this kind of sentence.

Because after cambiar de, Spanish often uses a bare noun with no article.

So:

• cambiar de ropa
• cambiar de tren
• cambiar de idea
• cambiar de casilla

This sounds natural in Spanish. Adding an article, like de la casilla, would usually change the meaning and make it sound like you are referring to one specific square in a different way.

In this sentence, yes, casilla means a square on the board.

More generally, casilla can also mean a small box or cell, such as:

• a checkbox on a form
• a square on a grid
• a board-game space

So it is a broader word than just chess vocabulary.

Grammatically, yes: it is present tense.

But in Spanish, the present can be used in several ways depending on context:

• a general fact: The pawn changes square
• a live description: The pawn moves to another square
• a narrative present: describing actions vividly as if they are happening now

So the exact English translation may vary a little depending on the situation.

Normally, no. Cambiarse is usually used when the subject changes itself, often with things like clothes:

• Se cambia de ropa = He/She changes clothes

For a chess piece, the normal form is simply El peón cambia de casilla.

It could, but it would not mean exactly the same thing.

• El peón mueve... usually means the pawn moves something, because mover is often transitive.
• To say the pawn moves, you would more naturally say El peón se mueve or use a chess-specific phrasing.

So cambia de casilla works well if the idea is it changes square.

You would make both the noun and the verb plural:

• Los peones cambian de casilla.

That means The pawns change square / move to different squares.

Notice:

• el → los
• peón → peones
• cambia → cambian