英语情景对话：
A: Who is on duty?
今天谁值班？
B: I am, sir.
我，先生。
A: Your duty is to keep a check on those prisoners from now on.
你的任务是从现在可是监视这些犯人。
B: Yes, sir.
是，先生。
习语：
keep a check on
解词释义：
check是名词，意思是“检查（以确保某事物正确，安全，满意或处于良好状况）”。Keep a check on直接中文译文就是“对......一直保持检查”，意思即“监视”。
支持范例：
We had been kept a check on since nightfall.
从黄昏以来，我们就遭到监视。
They have keeping a check on John in case he escapes.
他们严密监视他，惟恐他会逃脱。
A policeman in plain clothes have kept a check on the criminal all day.
有个便衣警察整天秘密监视那个罪犯。
A policeman with plain clothes stood a dozen paces behind me, keeping a check on me.
一个穿便衣的警察站在我后面十几步远的地方监视着我。
英语学习笔记：
1. on duty：在上班，在值班
Eg. The rule is that someone must be on duty at all the time.
按照规定，任何时候均需有人值班。
Eg. Some nurses at the hospital are on duty all night.
这个医院的一些护士整夜值班。
Eg. A policeman must not smoke while he is on duty.
警察当班值勤时不得抽烟。
Eg. Only one doctor is on duty today the other doctor is off duty.
今天只有一个医生值班，另一位下班了。
2. from now on：从现在开始，今后
Eg. He's left school and will go to work from now on.
他已经中学毕业，从今以后就要工作了。
Eg. From now on your movements will be the subject of close attention.
从现在起，你的一举一动将成为密切注意的目标。
Eg. This is the last time I go with you, from now on you must go alone.
这是最后一次我陪你了，以后你得自已去了。
Eg. I want you to be on your guard and watch your step from now on.
我要你注意检点自己，从此以后，步步当心。
