Learn French: Le dernier acte va commencer dans un quart d'heure. - The last act is going to start in a quarter of an hour.

In French, use dans with a time expression to say when an event will start in the future (15 minutes from now). If you used en, it would mean the act takes exactly 15 minutes to finish playing!

Why is it d'heure instead of de l'heure?

Fractions and quantities in French, like un quart (a quarter), are followed simply by de (which becomes d' before the silent H in heure). You don't need the word 'the' (l') here, so it is literally 'a quarter of hour.'

Why use va commencer instead of a regular future tense word?

This is the near future (futur proche), made with the present tense of aller (to go) and the infinitive commencer (to start). It translates directly to 'is going to start.' French speakers use this very often for events happening soon, making it a perfect match for dans un quart d'heure.

I thought adjectives usually go after the noun. Why is it dernier acte?

Most French adjectives do go after the noun, but adjectives describing order in a series (like first, next, or last) go before the noun. Since this is the 'last' act, dernier comes before acte.