1.spy作名词时意思是“间谍”,转化为动词意思是“窥探”,引申可指“观察”。
2.spy既可用作及物动词,也可用作不及物动词。用作及物动词时,可接名词、代词作宾语。有时也可接由现在分词充当补足语的复合宾语。用作不及物动词时,通常与介词或副词连用。

spy into(v.+prep.)
调查 look into
▲spy into sth
Tom began to spy into a complicated affair.汤姆开始调查一件复杂的事件。
The old lady likes spying into other people's affairs.这个老太太喜欢窥探他人的事情。

spy on(v.+prep.)
窥探; 监视 pry about; peep at
▲spy on sth
She often spies on her neighbours.她经常窥探邻居。
He was paid by the police to spy on other students.他受警察雇佣监视其他学生的行动。

spy out(v.+adv.)
察出 examine; look into
▲spy sb/sth⇔out
The detective spied out the suspected thief.侦探察出了被怀疑的贼。

用作及物动词
▲S+～+n./pron.
They left at daylight in order to spy the way.他们在黎明出发,目的是侦察出一条路。
He was the first to spy the horseman in the distance.他最先看到出现在远处的骑者。
用作宾补动词
▲S+～+n./pron.+v-ing
Our look-out spied a man hiding in some bushes.我们的哨兵察觉有人藏在树丛中。
We spied them coming through the gate.我们发现他们穿过这道门。

［词源］＜古英语spien＜古法语espier(间谍)

We have a spy among us.我们中间有一个间谍。
It seemed that she was a spy because she knew what everyone was doing.她似乎是个密探,因为她知道每个人的举动。