Now for the last one : SUR 

It's completely different, it means on something, sometimes physically, sometimes not, let me give you examples ;)

Examples:

- The cat is on the table = Le chat est sur la table. (how original) --> on, above
- The books are on the bookshelf = Les livres sont sur la bibliothèque. --> Note that if you used "in the bookshelf" instead, you would have translated it into : "Les livres sont dans la bibliothèque" which has the same meaning that in English. 
- I'm on the case = Je suis sur l'affaire. (it also works for this hehe)

• A = well it could mean many things... hmm.. I think most of the time it means at or to 

Examples:

- I'm going to the beach = Je vais à la plage.
- I'm at school = Je suis à l'école.
- I'm in school = Je suis à l'école. --> Here, saying I'm "in school" means you're currently studying at school as your main occupation right ? You're not saying you're INSIDE school like physically so that's why you're using à and not dans. 

In French there is no difference for these 2 cases.

So to sum things up, 
DANS = inside of something (object, building, abstract entity (like a group), a place)
EN = inside a country + time/transport/explanation/simultaneity preposition.
A = at a place, in a more generic way (at the TV, at the beach, at school, at the swimming pool (note the difference between at and in the swimming pool for respectively à and dans ;))

Hey John Paul ! 

Here we can group these 4 words in 3 categories for each of which there is a "corresponding word" in English. Obviously, French has a lot of subtilities so i'll try my best to explain it.

• Dans & En = in/into. Here I've got a very nice table showing in which case you use dans and in which case you use en. Actually EN has many other meanings so be sure to check that table.
https://francaisaguia.files.wordpress.com/2012/11/263697_371867119570025_686574598_n-1.jpg

Examples:

- I'm in the building = Je suis dans le bâtiment.
- The cat is inside the box = Le chat est dans la boîte.
- You're in my group = Tu es dans mon groupe.
- I'm in the train = Je suis dans le train.

- I'm in Belgium = Je suis en Belgique
- I'm going there by car = J'y vais en voiture.

The table is pretty self-explanatory for the word en which is a little more subtle than dans