A - Use "then" if you want to talk about the order of events. Use "so" if you want to talk about a result or reason to do something. Use "so" in your sentence. It's logical and obvious that someone would first want to improve his/her French and then think of going to France after. There's no need to indicate the order of events here. The result and the reason, which is what "so" indicates, is much more like something someone would want to say.Originally Posted by Emanuelli
B - I would choose one or the other. Using "with" can mean that you simply work in the same company or place as Paul, or that you actually work together with Paul on the same things within the company or place. Of course, "together" just means that you share the same work in some way. I don't think there is any need to say "together with" in this particular case, though it wouldn't be wrong. What you really mean by "together" and "with" would be accounted for in the entire context.
C - I would write some example sentences with the nouns you are not sure of. Someone can check them to see if you understand the idea of countable and uncountable nouns in English.