He needs to be kept in line.
要好好管束他一下。
keep sb. in line直译过来就是：“让某人排队”，这个短语的正确意思是：“管束，守规矩，让某人老实点”。因此，当美国人说"He needs to be kept in line."时，他/她要表达的意思就是:"He needs to discipline himself."、"He sould be taught to behave himself."。
情景对白：
Benjamin: My assistant always does what he thinks right, and he never cares about others.
本杰明：我的助理总是按照自己的想法去做，从来不管别人的感受。
Boss: He is unaware of his behaviour. He needs to be kept in line.
老板：他太不注意个人行为了，要好好管束他一下。
搭配句积累：
①I agree with you. Like teacher, like student.
同意，正所谓严师出高徒。
②I'm mad at him.
他快把我给气死了。
③He is going too far!
他太过分了！
④Otherwise, he will cause trouble sooner or later.
否则，他迟早会惹麻烦的。
单词：
discipline vt. 自律
Order and discipline have been placed in the hands of headmasters and governing bodies.
维持秩序和纪律的工作已交接给了校长和管理机构了。
It was that image of calm and discipline that appealed to voters.
正是那个冷静、自律的形象打动了选民。
True discipline is self-discipline.
真正的纪律是自律。
