Learn French: La pièce va commencer dans une semaine. - The play is going to start in a week.

In French, dans followed by a time expression means 'in [amount of time] from now'. Because the play starts a week from this moment, dans is the right choice. You would use en if you were talking about how long it takes to complete an action (like 'I read the play in a week').

I have seen pièce used to mean 'room' or 'coin'. How do I know it means 'play' here?

Pièce is a versatile word! It can be a room (une pièce de la maison), a coin (une pièce de monnaie), or a theatrical play (une pièce de théâtre). Context is key: since this sentence talks about something that is 'going to start' (va commencer), a play makes the most sense.

Is va commencer a specific future tense?

Yes, this is the near future (futur proche). You make it by taking the present tense of aller (here, va for 'is going') and adding the infinitive of the action verb (commencer for 'to start'). It works exactly like the 'is going to' structure in English.

If I wanted to say the play lasts for a week, would I still use dans?