is a command, but please is usually implied. you are telling them to wait, which is more rude than asking.
Quote:You do wait here , I'll go.
replace "do" with "will" and the sentence becomes proper and has the meaning you want. you are telling them to wait AND assuming that they will follow your command, which is more rude than just telling them.
Quote:'Don't let's give up.'
I would say this is just flat out bad English...I've never heard it said this way either
Quote:They are to go by train.
is usually stated as "they will go by train"