Once引导时间状语从句的情况如果逐句和从句的主语一致,可以把从句的主语和be动词省略,那如果从句为一般现在时没有be动

你说的4种情况分别如下：
1.Once you are in America,you will know how developed the country is.
Once in America,you will know how developed the country is.
2.Once you climb onto the top of the mountain,you will overlook the whole city.
Once climbing onto the top of the mountian,you will overlook the whole city.
3.Once you have understood the rule,you will obey it.
Having understood the rule,you will obey it.
4.Once you can speak English,you will feel very happy.
不能省.