spy ·phrasal verb

情报官员监视嫌疑人的活动数月之久未被发现。

The intelligence officer ___spied ___on the suspect's activities for months without being detected.

公司有时会监视竞争对手以揭露商业策略。

Companies sometimes ___spy ___on their competitors to uncover business strategies.

未经同意监视他人的个人通信是不道德的。

It is unethical to ___spy ___on someone's personal communications without consent.