as green as grass 少不更事，无生活经验，浑然无知

【例句】
When he left his school and started his first job, he was as green as grass.
当从学校毕业，并开始了他的第一份工作时，他太缺乏经验了。

Those who are as green as grass can't be leaders of groups.
那些缺乏经验的人是不能当任小组领导工作的。