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?