À or Dans or En? (I think)
This is according to how I see Duo's teaching me. Correct me.
This is not a continuation of my post in differentiating à and de, I asked to point out if there are some errors there and no one responded. Also, I asked if I should continue that thing in more aspects and nobody responded. Anyway, thank you for those who gave me lingots ;)
This does not cover the other uses of à except for pointing locations (I have posted for other uses of à in comparison with de, idk if i should continue that) and the adverbial pronoun use of en.
À is referring to a specific point.
Elle est à Nantes.
J'habite à Manille.
Dans refers to something with definite 3D boundaries, physical boundaries. Like a box or room. Also, dans COULD come with an article.
Le mouton est dans la boîte.
Je danse dans le noir.
En, however refers to something with boundaries but INDEFINITE. Like land borders, there exists a concept of "borders", but when you go to a certain place no such line exists. (Well, ouais, the DMZ)
Wait, à could take cities as well, then, what makes it different to en?
In the examples, I used Nantes and Manille, those cities were definite. You know the name of the city, or otherwise it was implied. A differentiation of it to à was en's indefiniteness could even extend to names.
Je suis en classe. (What kind of class? Math class? Chemistry class? History class? It wasn't specified.)
Je vais en île (Which island?)
Compare the last example to « Je vais à Australie. », à is used because the island was specified.
As you learn, I recommend thinking about prepositions that go with proper place names (names of countries, islands, cities, states, etc.) as its own separate topic. You will find that if you are travelling to a city or an island, for example, the correct preposition is usually 'à' ("à Londres"); to a masculine country, it's usually 'au' ("au Maroc"); to a feminine country, it's usually 'en' ("en Inde"), but there are many exceptions. (By the way, in your last example, it should be "Je vais en Australie.")
So putting proper place names aside, the rest of your question seems to be about 'en' vs. 'dans', the issue being that they both seem to mean 'in', is that right? Well, mostly 'dans' takes a noun with an article ("dans la boîte") and 'en' takes a noun without one ("en classe") (remember that nouns without an article are much rarer in French than in English). The other easy-to-spot distinction has to do with time expressions: something that happens 'en une heure' takes an hour to accomplish, while something that happens 'dans une heure' will begin one hour from now.
But alas, because nothing can be simple, you can also have "Je vais dans une île" (or "sur une île", but not "en île"). This is more of a kind of exception, since in this case 'dans' doesn't translate to 'in' at all because in English we say "on an island", not 'in'.
Clear as mud?