How to use the prepositions en and dans in French?

The prepositions en and dans both mean "in" in French, and can both express time and location. But they are not interchangeable; their usage depends on both meaning and grammar.

En expresses the length of time an action takes. Note that this means the verb is usually in the present or past.

Il a lu le livre en une heure. -> He read the book in an hour.

En is used to express the month, season, or year in which an action takes place.

Nous voyageons en avril. -> We travel in April.

En can mean "in" or "to" when followed directly by a noun that doesn't need an article.

Il est en classe. -> He's in school.

Dans indicates the amount of time before which an action will occur. Note that this means the verb is usually in the present or future.

Nous partons dans dix minutes. ->  We're leaving in 10 minutes.

Dans refers to something that occurs within or during a decade.

Dans les années soixantes...  ->  In the sixties...

Dans means "in" a location when followed by an article plus noun.

Il est dans la maison. -> He's in the house.

Dans also means "to" or "in" with some states and provinces.

J'habite dans le Maine. -> I live in Maine.