you cannot burn the candle at both ends.
蜡烛不能两头点，精力不可过分耗。
you cannot eat your cake and have it.
鱼与熊掌，不可得兼。
you can take a horse to the water but you cannot make him drink.
强扭的瓜不甜。
you may know by a handful the whole sack.
由一斑可知全貌。
you never know what you can till you try.
是驴子是马，拉出来遛遛。