1. 观察者；观测者；目击者
    You can refer to someone who sees or notices something as an observer .

    e.g. A casual observer would have taken them to be three men out for an evening stroll...
           乍一看的话，还以为他们三人是晚上出来散步的。
    e.g. Observers say the woman pulled a knife out of the bunch of flowers and stabbed him in the neck.
           目击者说那名女子从花束中拔出一把刀来，扎进了他的颈部。

2. 观察家;评论员
    An observer is someone who studies current events and situations, especially in order to comment on them and predict what will happen next.

    e.g. Observers say the events of the weekend seem to have increased support for the opposition...
           观察家说周末发生的事件似乎为反对派赢得了更多支持。
    e.g. Political observers believe that a new cabinet may be formed shortly.
           政治观察家认为新内阁可能很快就会组建。

3. （监督重大事件或局势的）观察员
    An observer is a person who is sent to observe an important event or situation, especially in order to make sure it happens as it should, or so that they can tell other people about it.

    e.g. The president suggested that a UN observer should attend the conference.
           总统建议联合国应派一名观察员参加本次会议。