一般而言，英文里尽量使用主动语态，主动语态通常更为直接、有力，比如：
I shall always remember my first visit to Boston.
这样就比下面这个句子要好：
My first visit to Boston will always be remembered by me.
被动语态显得拖沓，不够简洁，如果把“by me”省略，又会使得指代不够明确，是作者还是其他人always remember this visit?
当然，这并不是说被动语态完全不能用，根据强调的对象不同，也可以选择被动语态，比如：
The dramatists of the Restoration are little esteemed today.（强调 dramatists of the Restoration）
Modern readers have little esteem for the dramatists of the Restoration.（强调 Modern readers ）
不过总体来说，一个平庸的句子（比如含有“there is”或“could be heard”）变成主动语态（使用及物动词）后会更加生动、有力。以下例子可以说明：
There were a great number of dead leaves lying on the ground. > Dead leaves covered the ground.
At dawn the crowing of a rooster could be heard. > The cock's crow came with dawn.
The reason he left college was that his health became impaired. > Failing health compelled him to leave college.
It was not long before she was very sorry that she had said what she had. > She soon repented her words.
以上例句变成主动语态后，不仅更加有力，而且也更加简短了。
但是，在科技论文里情况却有所不同，主动语态和被动语态的选择一般考虑以下几个因素：
1）读者是否需要知道动作（action）的实施者（actor）？
在Methods部分，实验本身才是重点，至于是谁做的实验一般不重要，所以一般用被动语态。比如：
The hippocampal MAP-2 expression was measured by immunohistochemistry.
但是要用主动语态也并非不可，如果用主动语态的话，就会经常使用人称代词，比如“We”，这就涉及到以下第二点。
2）总是使用“We”显得重复而不够正式。
在Methods部分如果用主动语态的话，就要经常使用“We”作为主语，为了避免重复，一般还是以被动语态为主。
比如：
We calculated overall survival and progression-free survival by the Kaplan-Meier method
不如写成：
Overall survival and progression-free survival were calculated by the Kaplan-Meier method.immunohistochemistry.
3）用被动语态也存在一些问题，比如主语过于冗长。
比如：
A total of 100 patients with chronic hepatitis B admitted to this hospital from June 2012 to June 2014 were included in this study.
可以写成：
This study included 100 patients with chronic hepatitis B admitted to this hospital from June 2012 to June 2014.
总之，主动语态和被动语态的选择不是绝对的，一般考虑句子需要强调的重点在哪里，以及句子是否简洁有力。
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