If i stay in a nice hotel in paris and i book a single room, can my girlfriend stay with me for the whole stay

No, I believe you would be charged for the room, not the number of people. Unless things are different in Paris, you can have your girlfriend stay in the room with you for no additional fee. I mean, whose business is it, anyway? Hope this helps and have fun in the City of Looove! ;-)

If you get a single room, only one person is allowed to stay there. You will need to get a double or a twin room. Hotels know that some people try to do that to save money, so they watch out for that. When you rent a hotel room, it is like entering into a contract, and you can get a lot of trouble for doing something like that, you would be very lucky if they only charged you for a double room and then kicked you both out. You shouldn't need to try do something like that, if you do enough research, you can find very nice hotels in Paris cheaply, just do some internet searches instead of potentially putting your girlfriend and you in a very bad situation.