"It's imposible for them to master English"
I learnt you say "It's impossible for them to master English" or "It's impossible that they should master English."
Here're a couple of questions.
1) In the second sentence, why do you use "should" there? If it's removed, doesn't the newly made-up sentence make any sense?
2) I know you never say something like "They are impossible to master English", but I wonder what this sounds like... You say "He is hard to please", why not "They are impossible to master English"?