"You should not lose to her." is ordinarily used BEFORE the game. However, "S ní bys neměla prohrát." sounds, to my ear, as a statement AFTER the defeat, as "You should not have lost to her", or "You were not supposed to lose to her". Which English translation conveys the exact meaning of the Czech sentence?
If it is used before the game, then, doing this exercise the other way around, can I translate the straightforward English expression "You should not lose to her." with the straightforward Czech expression "S ní nemáš prohrát"? What would conditional give me here, only a degree of politeness?
Well, "S ní nemáš prohrát" is quite strange.
"S ní bys neměla prohrát" expresses both meanings of "should not" - the likely one: "it's unlikely that you will lose to her" as well as the sort of warning: "it would be unadvisable to lose to her".
The plain "S ní nemáš prohrát" is straight "You're not supposed to lose to her" and it's a bit bizarre. It's like "your skill is so superior that you'd have lose deliberately, otherwise you always win by default, and remember, we agreed that you wouldn't deliberately lose to this woman." ...so yes, "You shouldn't lose to her" can also mean this, but it's quite marginal, isn't it?