Q:

The question about the English articles.  A/an or The? 

I've been looking for an answer to my question for a long time, but I've got no results.  I've read plenty of grammar books but haven't found the straight and clear answer for the question in the topic "Why do you say "to/at/in the zoo, to/at/in the shop, to/at/in the river, to/at/in the cinema, to/on the roller coaster etc.?  Why don't you use indefinite articles when a place, a thing (like "roller coaster) is not specific?" 
Can somebody explain to me why we still say "I go to the shop", "I'am at the shop" even if we don't go to or at a specific shop or we mention it for the first time?  Can we say "I go to a shop", "I am at a shop"? 
Why do we still say "I go to the zoo", "I am at the zoo" even if we don't go to or at a specific zoo, or we mention it for the first time?  Why don't we say "I'm at a zoo", "I go to a zoo"? 
The same questions about to/at/in the river, to/at/in the cinema, to/on the roller coaster etc. 
I'm really struggling to understand it.  I think I'm not alone. 
When answering the question, please, don't use things like "we just use".  I need a clear, grammarly based explanation with all the details. 
Or tell me about the books that I could use to clarify the matter by myself. 
Sorry for the grammar and punctuation mistakes in my text, no one is perfect. 
Thanks to everybody who is trying to answer the question. 

A:

The reason why we use the definite article for unspecified places is for one of two reasons: because we are confident the listener will assume which specific place we are talking about; or because it doesn't matter. 
For example, I live in a town that has a zoo.  So when I say "I went to the zoo", it would be assumed by most people that I mean the zoo in my town.  If I said "I went to a zoo", the lack of specificity would almost certainly prompt someone to ask "which zoo?".  If I did actually go to a zoo other than the one in my own town, I would probably name it, for example "I went to Edinburgh Zoo". 
People say "I'm going to the shop" because they have a specific shop in mind, but they don't need to specify because it is either obvious (perhaps there is a shop at the corner of the street that is the default choice due to its convenience) or because it doesn't matter.  I usually say "the shop" to refer to my local convenience store.  If I am going somewhere further afield I would specify either "the supermarket" or call the shop by name.