难度：高级
论文的引言和文献综述部分必须包括研究背景。成功频繁发表文章的作者知道如何有效使用动词时态，并将其作为一个强有力的说服手段。
让我们看下面的例子：
Gupta et al. (2012) show // showed that lower vitamin D levels in children with severe, therapy-resistant asthma (STRA) are // were associated with worse asthma control and lung function.
(Gupta等人（2012）表明在难控哮喘的重症患儿中，维生素D水平越低，哮喘控制和肺功能越糟糕。)
这两个动词(show,associate)是报告研究结果的标准形式。第一个动词表示报告（“显示”、“证实”、“建议”等），而第二个动词涉及到句子的内容。我们看一下现在时和过去时, 看一下该使用哪种动词时态。
第一个动词 show//showed
第一个动词使用过去时态是恰当的、没有争议的，我们建议使用过去时，因为研究是在过去开展的。
正确而自然: Gupta et al. (2012) showed that… (Gupta 等人(2012) 表明…)
如果使用现在时并不是不可以：它表明“现在已发表的、可供阅读的研究指出......”。但是会有点儿不自然，所以请谨慎使用。
正确但不自然: Gupta et al. (2012) show that… (Gupta 等人(2012) 表明…)
第二个动词 are//were associated
使用过去时态，可将调查结果定位在实验时期，而使用现在时表明至今研究结果仍是真实、可适用的。are或were都可以使用，但现在时语气更为强烈，使用现在时表明对调查结果的强烈肯定。
正确：…lower vitamin D levels in children with STRA were associated with worse asthma control and lung function.
(在难控哮喘的重症患儿中，维生素D水平越低，哮喘控制和肺功能越糟糕。)
[这里使用过去时，因为这是Gupta等人在过去的研究。]
正确并强烈肯定: …lower vitamin D levels in children with STRA are associated with worse asthma control and lung function.
(难控哮喘的重症患儿中，维生素D水平越低，哮喘控制和肺功能越糟糕。)
[这里使用现在时， 表明Gupta等人的研究直到今天仍是可信的。]
我们甚至可以使用过去时态含蓄地否认过去的调查结果。
Smith et al. (2008) found that vitamin D levels had no relation with asthma severity or recurrence. However, Gupta et al. (2012) reported a contradictory finding, saying that lower vitamin D levels in children with STRA are associated with worse asthma control and lung function.
(Smith等人（2008年）发现，维生素D水平与哮喘的严重程度或复发无关。然而， Gupta等人（2012年）报道了一个相反的调查结果:对患有STRA的儿童，维生素D水平越低，哮喘控制和肺功能越糟糕。)
[Smith 等人说没有联系，但是我们相信，事实上是有联系的。]
若要更有力地报告调查结果，可以完全省略第一部分（“ Gupta等人表明...”），因为有引用，读者会知道这是在报告一个调查结果。例如：
Lower vitamin D levels in children with STRA are associated with worse asthma control and lung function (Gupta et al., 2012).
(在难控哮喘的重症患儿中，维生素D水平越低，哮喘控制和肺功能越糟糕 (Gupta等, 2012)。)
总结：
- 使用“showed”, “demonstrated”, “suggested”等报告类动词(reporting verbs)时，过去时会更自然。
- 现在时和过去时都可以用来描述调查结果，现在时的表达更为强烈。
- 为了表示语气坚定，可以删除报告类动词。
Download Tip Here: 
引言中的过去时和现在时
Uni-edit English Writing Tip 007: Using Past Tense and Present Tense in your Introduction Section
Difficulty: Advanced
The Introduction and Literature Review sections of your paper must include background to your research. Authors who publish successfully and publish often know how to use verb tense effectively as a powerful persuasive tool.
Let’s take the example below, whose two verbs are a standard format for reporting findings. There are two verbs: the first verb has a reporting function (“to show”, “to demonstrate”, “to suggest”, etc.), while the second verb relates to the contents of the sentence. We will look at present tense and past tense.
Which verb tense should we insert?
Gupta et al. (2012) show // showed that lower vitamin D levels in children with severe, therapy-resistant asthma (STRA) are // were associated with worse asthma control and lung function.
First verb - show//showed
Using past tense for the first verb is uncontroversial and recommended. It is appropriate because the research was conducted in the past.
Correct & Natural: Gupta et al. (2012) showed that…
Using the present tense is not forbidden: it suggests “the study, which is published and available to read at present, says…”. However, it can sound affected: use with caution.
Correct & Slightly Strange: Gupta et al. (2012) show that…
Second verb – are//were associated
Using past tense localizes the findings to the time of the experiment, while using present tense suggests the findings are true and applicable even now. You can use both, but the present tense sounds stronger. Using the present tense for the second verb will be perceived as a strong assertion of the findings!
Correct: …lower vitamin D levels in children with STRA were associated with worse asthma control and lung function.
[This was a finding of Gupta et al.’s study in the past.]
Correct and stronger assertion: …lower vitamin D levels in children with STRA are associated with worse asthma control and lung function.
[This finding of Gupta et al. holds true even today.]
You can even use the relative weakness of the past tense to implicitly dismiss past findings.
Smith et al. (2008) found that vitamin D levels had no relation with asthma severity or recurrence. However, Gupta et al. (2012) reported a contradictory finding, saying that lower vitamin D levels in children with STRA are associated with worse asthma control and lung function.
[Smith et al. said there was no relation, but we believe that, in reality, there is a relation.]
To report findings most strongly, you can omit the first part (“Gupta et al. showed…”) entirely! The readers will know you are reporting a finding because of the presence of a citation.
Lower vitamin D levels in children with STRA are associated with worse asthma control and lung function (Gupta et al., 2012).
To sum up:
- Past tense is more natural for reporting verbs like “showed”, “demonstrated”, and “suggested”.
- Present tense and past tense are both okay for describing findings: present tense is stronger.
- You can even omit the reporting verb altogether for a very strong assertion.