Metti il segnalino sulla casella.

Metti is the tu imperative form of mettere, so it means put! when speaking to one person informally.

• mettere = to put
• metti = put! / you put

So Metti il segnalino sulla casella is an instruction addressed to one person, like in a game.

It can be both, depending on context.

• As a statement, metti can mean you put
• As a command, metti! means put!

In this sentence, it is understood as a command because the sentence is giving an instruction.

Italian usually uses an article where English might not. Il segnalino literally means the token / the marker / the playing piece.

In game instructions, Italian often includes the article even when English might simply say Place your token or Place a token. The exact English article may vary in translation, but in Italian il is completely natural here.

Segnalino usually means a small marker, token, counter, or playing piece, especially in board games.

It comes from segnale or segnare ideas related to marking/signaling, and the ending -ino often gives a sense of something small. So segnalino is basically a little marker.

Because sulla is the normal contraction of su + la.

• su = on, onto
• la = the
• su la → sulla

This kind of contraction is very common in Italian:

• su + il = sul
• su + lo = sullo
• su + la = sulla
• su + i = sui
• su + gli = sugli
• su + le = sulle

So sulla casella means on the square/space.

Here casella means a square, space, or box, depending on the context.

In a board game, it usually means the space on the board where a piece goes. In other contexts, casella can also mean:

• a box on a form
• a square in a grid
• a checkbox area

So in this sentence, it most likely refers to a board-game space.

Because sulla means on the square/space, which fits a board-game idea: the token is placed on a space.

• sulla casella = on the square/space
• nella casella = in the box/inside the space

With board games, Italian commonly uses su because the piece rests on top of the board space, not inside something.

Yes, that is possible. Italian word order is more flexible than English.

• Metti il segnalino sulla casella is the most neutral order.
• Metti sulla casella il segnalino is also possible, but it may sound slightly more marked or focused on where to put it.

The original version is the most natural basic instruction.

It is informal singular, because metti is the command used with tu.

If you wanted a formal command to one person, you would say:

• Metta il segnalino sulla casella.

If you were speaking to more than one person:

• Mettete il segnalino sulla casella.

With the informal tu form, the negative imperative uses non + infinitive:

• Non mettere il segnalino sulla casella.

This is different from the positive command:

• Metti il segnalino sulla casella.
• Non mettere il segnalino sulla casella.

A rough pronunciation guide is:

MEHT-tee eel seh-nya-LEE-no SOOL-la ka-ZEL-la

A few useful points:

• tt in metti is pronounced more strongly than a single t
• gli is not in this sentence, so nothing tricky there
• gn in segnalino sounds like the ny in canyon
• zz is not present; casella has a clear z-like/s sound depending on accent, but learners can safely aim for ka-ZEL-la

Yes, depending on the game or context. Possible alternatives include:

• pedina = playing piece / pawn
• pezzo = piece
• getto = token, counter in some contexts

But segnalino is a very natural general word for a marker or token used to indicate position.