Advanced authors use tense shifts to control the tone and make the text more interesting and engaging to read. There are several specific reasons to shift the tense in the Introduction Section:
- indicate the timeline of the story;
- emphasize some research over other research; and
- indicate which research results are no longer considered current or relevant.
Although in general tense would not change within a sentence, it does sometimes make sense and should be done. The example text in the past perfect tense section above is one such instance, and here is another example:
Although it has long been thought that pineapples are not nutritious (e.g. Vine et al. 2001), this new study proves it conclusively (Whyte and Blythe 2016).
The first part of the sentence above is in present perfect tense, and the second part is in simple present tense. Thus, tense can be shifted within a sentence if necessary to indicate the chronological order of different studies.
In a paragraph in the literature review part of the Introduction Section (or if you are reviewing literature in the Discussion Section), the tense should largely be consistent, but should be changed to indicate different levels of confidence in the studies under review.
In the Introduction Section as a whole, most of the literature review part of it should typically be in present perfect tense. There could be a few instances of simple past tense, where the author’s confidence in the studies is lower, and one or two instances of simple present tense, where the author’s confidence in the knowledge is very strong. It is less likely that there would be any past perfect tense, but past perfect tense might be used if the chronological order of studies is important to convey.
It is important to be aware that if all of the literature review is written in one tense (particularly in simple past tense), then the reader cannot tell anything about how the author feels about any of the findings they are reporting. The different implications of the tenses thus only become apparent when the tenses are mixed to some degree in the text.
The final part of the Introduction Section, however, which is the statement about the present study, will generally be in either simple past tense or a mixture of simple present tense and simple future tense.