Generally this simple distinction en = in and dans = inside is enough to know when to use one or the other. En is commonly designing open areas (e.g. en France, en ville) while dans is designing boxed/closed/limited areas (e.g. dans le centre ville, dans le magasin). Note that en sometimes refers also to a mean(*) : - je fais de la plongée en mer / en piscine / dans le lac. You have to understand this as : - I'm doing sea diving /pool diving / I'm diving in(=inside) the lake. In case you were wondering why en is used with a pool which is smaller than a lake. (*) to be more specific it is when you can ask a question like : what kind of ... ? What kind of diving are you doing ? sea diving. De la pongée en mer. What kind of fishing are you doing ? river fishing. De la pêche en rivière. What kind of trek are you doing ? in mountain/ in forest. De la randonnée en montagne/ en forêt. etc... Unfortunately there are exceptions, we say dans le désert and en zone désertique. For some reasons, désert is considered as if you are always inside the desert, as in a boxed area.