Learn French: Le train arrive dans un quart d'heure. - The train arrives in a quarter of an hour.

In French, the phrase for "a quarter of an hour" is a fixed expression: un quart d'heure. You don't need the word une (a/an) here. It translates literally more like "a quarter of hour".

Does dans mean "in" or "inside"?

Dans usually means "inside" when talking about physical locations (like dans la maison). But when talking about time, it means "in" a certain amount of time from now, just like in English: dans un quart d'heure (in a quarter of an hour).

Why is it d'heure instead of de heure?

The letter h in heure is a "silent h", meaning the word acts like it starts with a vowel sound. Because of this, the word de (of) drops its e and attaches to the next word with an apostrophe to make it flow better: d'heure.

How does un quart work when telling the actual time on a clock?

When telling time, like "It is 3:15", you say Il est trois heures et quart (literally: It is three hours and quarter). But when talking about a duration of time, like in this sentence, you use the full phrase un quart d'heure.

Can I use the present tense () for something happening in the future?