㈠逗号①何时不可省略？②可省可不省③必须省略？

㈡下面有两个关于逗号省略的例子:

1.In 1988,75 Nobel Prize winners said that if mankind is to survive we must go back 25 centuries to make use of the wisdom of Confucius.

2.After graduation，she reached a point in her career where he needed to decide what to do. 

疑问:1句从句放主句前不是应该用逗号连接吗？为什么没逗号?是在宾语从句里的缘故吗？

2句中的逗号是否可以省略？

希望老师可以给一个系统性的解答，不胜感激!