Learn Hardcore Catalan: Doncs, tira el dau.

Doncs is a discourse marker, often meaning something like well, so, or well then depending on context. It helps move the conversation along.

In Doncs, tira el dau, it gives the sentence a natural spoken feel, as if someone is saying:

Well then, roll the die.
So, go ahead—roll the die.

It is not the main action of the sentence; the main command is tira el dau.

Why is it tira and not tires?

Because tira here is the imperative form, used to give a command to tu.

The verb is tirar = to throw / to roll.
Its tu imperative is tira.

Compare:

tu tires = you throw / you are throwing → statement
(tu) tira = throw! / roll! → command

So tira el dau means roll the die!, not you roll the die as a plain statement.

Why is the subject missing? Where is tu?

In Catalan, subject pronouns are often omitted when they are clear from the verb form. Since tira already shows that the command is directed at tu, you do not need to say tu.