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.
Download Tip Here: 
Present Perfect Tense and Past Tense in your Introduction
END OF TIP