Why do I use en here: Mon frère habite en Françe
But here I use dans here: Je suis dans le jardin
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Dans is used to express that you are inside of something
En however defines a location which is more global such as a continent, a feminine region (country or whatsoever)
But, you won't use en for masculine regions
Obviously, these concern the very basic usage of both words, they are used in plenty more situations. For instance, you wouldn't say Je suis dans la maison for I'm home, but rather Je suis à la maison. This, is just a glimpse of the numerous other ways these words can be tricky to use.