难度：高级
我们不知道听过多少次：“过去时表示事情发生在过去，而现在完成时不仅表示事情发生在过去，并对现在仍有影响。”那么研究论文的引言部分该使用哪种时态呢？难道不是所有过去的研究都跟现在相关？
是不是很迷惑？我们现在就为大家做一个简单的概括。
现在完成时
现在完成时通常用于有关领域发展趋势的宽泛表述。一些时间修饰词，如“最近”、“长期被视为”、“引起关注”及“引发批评”通常表明句子是现在完成时。因为这些事件未发生在特定的时间点，而是一个发展趋势。
正确：The contribution of agricultural production to greenhouse gas has long been debated.
（农业生产对温室气体的影响争论已久。）
正确：Recently, the idea that some crops leave larger CO2 footprints than others has gathered attention.
（近日，一些农作物的二氧化碳排放量大于其他农作物的观点已经受到了广泛关注。）
正确：Some researchers have argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes.
（一些研究人员认为，需要大量使用氮肥的农作物，如土豆，其CO2排放量大于其他使用较少氮肥的农作物，如豆类。）
在括号中注明具体的时间，也是现在完成时的一种用法：因为日期不是句子的语法成分。例如：
正确：Some researchers have argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes (2002).
（一些研究人员认为，需要大量使用氮肥的农作物，如土豆，其CO2排放量大于其他使用较少氮肥的农作物，如豆类。（2002））
一般过去时
若句子中指明了具体的研究日期，那么有必要使用一般过去时。这种情况下若使用现在完成时会显得不自然。
正确：Smith et al. conducted a 2-year field experiment in 2009 and 2010 in rice paddies to learn about the connection between rice production and greenhouse gas emissions.
（在2009及 2010年，史密斯等人在稻田进行了两年田间试验以了解水稻生产和温室气体排放之间的关系。）
正确但不自然：Smith et al. have conducted a 2-year field experiment in 2009 and 2010 in rice paddies to learn about the connection between rice production and greenhouse gas emissions.
在引言部分，若提及具体的作者名字，通常使用一般过去时。
正确： Smith et al. conducted a 2-year field experiment in rice paddies to learn about the connection between rice production and greenhouse gas emissions.
（史密斯等人在稻田进行了两年田间试验以了解水稻生产和温室气体排放之间的关系。）
正确： Bouwmann et al. (2002) argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes.
（Bouwmann等人（2002）认为，需要大量使用氮肥的农作物，如土豆，其CO2排放量大于其他使用较少氮肥的农作物，如豆类。）
说明：请注意，在上面的例子中， 当我们用“Bouwmann等人”而不是“一些研究人员”时，我们用的是现在完成时，这是因为“一些研究人员”不是具体的，而“ Bouwmann等人”是具体的。
同样的原则适用于：“我们”或“本研究的作者”，指的都是具体的研究人员，所以应使用过去时来描述过去的研究发现。
正确：We previously conducted an experiment to learn about the carbon footprints for various crops grown in Taoyuan County (2011).
（我们之前开展了一项实验以了解在桃园种植的各种农作物的碳排放量。（2011) ）
否定句
在否定句中，现在完成时更为常见的。在同时使用“考察”、“决定”、“透露”等动词时，更是如此。
正确：Whether reducing potato crops is a viable strategy to reduce greenhouse gas emissions has not yet been determined.
（是否减少土豆作物是一个可行的减少温室气体排放的策略尚未得到定论。）
想想看，如果某事从未被研究过，那就没有具体研究时间。例如，比较“A polar bear was born in the zoo yesterday (一只北极熊昨天在动物园出生)”和“A black polar bear has never been born (人们从未发现黑色北极熊)”。
正如之前所述，如果句子中有指明具体日期或作者，那么使用一般过去时是比较合适的。
正确: Smith et al. conducted a 2-year field experiment in rice paddies to learn about the connection between rice production and greenhouse gas emissions. However, they did not consider the effects of nitrogenous fertilizers.
（史密斯等人在稻田进行了两年田间试验以了解水稻生产和温室气体排放之间的关系，然而他们没有考虑氮肥的影响。）
总之：
- 若句中未指明具体时间和作者，则使用现在完成时。
- 用现在完成时表示趋势或尚无定论，反之，则使用一般过去时。
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引言中的现在完成时和过去时
Uni-edit English Writing Tip 006: Present Perfect Tense and Past Tense in your Introduction
Difficulty: Advanced
How many times have you been told the following: “the past tense denotes an event that happened in the past, while the present perfect tense denotes an event that happened in the past, which has present consequences.”? But, what about the Introduction section of research papers? Aren’t all of the past findings relevant to my present research?
Confused? So are we! Here is an easy guide.
Present perfect
Present perfect is often used for broad statements about trends in the field. Temporal adjectives like “recently” and “long considered” and words like “gather attention” and “draw criticism” often suggest the present perfect tense. This is because these events did not occur at specific point in time: rather, they are trends.
Correct: The contribution of agricultural production to greenhouse gas has long been debated.
Correct: Recently, the idea that some crops leave larger CO2 footprints than others has gathered attention.
Correct: Some researchers have argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes.
This usage of present perfect includes cases where the date—a specific point in time—is expressed in parentheses: because the date is not a grammatical part of the sentence.
Correct: Some researchers have argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes (2002).
Simple past
When the date of a study is expressed in the body of the sentence, past tense is necessary. In fact, present perfect sounds unnatural.
Correct: Smith et al. conducted a 2-year field experiment in 2009 and 2010 in rice paddies to learn about the connection between rice production and greenhouse gases emissions.
Unnatural: Smith et al. have conducted a 2-year field experiment in 2009 and 2010 in rice paddies to learn about the connection between rice production and greenhouse gas emissions.
In the Introduction Section, past tense usually applies when the specific authors are referenced by name.
Correct: Smith et al. conducted a 2-year field experiment in rice paddies to learn about the connection between rice production and greenhouse gas emissions.
Correct: Bouwmann et al. (2002) argued that crops that require heavy use of nitrogenous fertilizers, such as potatoes, leave a larger CO2 footprint than crops that require less, such as legumes.
Explanation: Notice that when “Bouwmann et al.” was “Some researchers” in the Present perfect explanation above, it was present perfect. This is because “some researchers” is not specific, whereas “Bouwmann et al.” is specific.
The same principles apply to you: “we” or “the authors of the present study” are specific researchers, so past tense should be used to report findings of your past studies.
Correct: We previously conducted an experiment to learn about the carbon footprints for various crops grown in Taoyuan County (2011).
Negative sentences
The present perfect is more common when the grammatical tense is negative (i.e. when “not” is used.) This is especially true with verbs like “investigated”, “determined”, “revealed”, etc.
Correct: Whether reducing potato crops is a viable strategy to reduce greenhouse gas emissions has not yet been determined.
Consider that if something has never been investigated, there is no specific time when it was not investigated: compare “A polar bear was born in the zoo yesterday” with “A black polar bear has never been found”.
That being said, again, the simple past is more appropriate if the specific date or authors are written in the sentence.
Correct: Smith et al. conducted a 2-year field experiment in rice paddies to learn about the connection between rice production and greenhouse gas emissions. However, they did not consider the effects of nitrogenous fertilizers.
To sum up:
- Use present perfect when the specific date or authors are not written in the sentence.
- Use present perfect to denote trends or lack of conclusions.
- Use past tense otherwise.
END OF TIP