我们都知道，英语中主动句和被动句的切换可以影响句子想表达的重点。例如下面两个句子所说的含义都一致(Joe 扇了the boy 一巴掌)，但是两句的重点却不同。主动句中，重点在主语(Joe); 而被动句中，重点在宾语the boy上。

主动句: Joe slapped the boy.

被动句: The boy was slapped by Joe

了解了句子的主被动差别后， 我们再来看看留学生论文写作中，到底该如何选择合适的句式~

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1. 主动句往往更加清晰，直接，准确;

√ Active voice: Joe found a relationship between stress and high blood pressure. (10 words)

而被动句则需要用更多的单词来表达清楚同样的含义， 这会让句子的表达显得赘述：

× Passive voice: The relationship between stress and high blood pressure was found in the study conducted by Joe. (16 words)

所以，为了让论文句式更加direct, clear, and concise，大家可以多多使用主动句式。

另外一个例子：

× The research which had been undertaken by the university was critiqued from an ethical point of view and was promptly retracted by them.

这里连续被动句的使用让句子变得赘述无比。所以，为了让句子看起来更加简洁，可以尝试多用主动句。

2. 但这并不意味着被动句就一无是处，在如下3种情况下， 被动句往往更优。

a) 当想强调动作的接收者时;比如，在写论文的methods 环节，往往用被动句，能把读者的注意力拉到实验对象上(主要信息)。

×We stored the bacteria in a test tube for one year. (active)

√The bacteriawere stored in a test tube for one year. (passive)

如果这里使用主动句，实验对象 (the bacteria) 就被放在了句子的次要位置，这不利于读者快速把握到methods 的核心。

b) 为避免使用第一人称时，用被动句

× I will discuss Kant’s philosophical theories in this essay. (active)

√ Kant’s philosophical theories will be discussed in this essay. (passive)

c) 当执行动作或者发表言论的人不确定是谁，或者不知道是谁时

× Someone ambushed the troops at night. (active)

√ The troops were ambushedat night. (passive)