Use past tense when reporting research (e.g. writing a literature review). One way to think about this is that the research reports about which you are writing have been written in the past: the research is finished and reported. If you are writing a research proposal, the tense you use will help distinguish between your proposed study and the research literature you draw upon. The APA Manual advises that "the past tense is appropriate when expressing an action or a condition that occurred at a specific, definite time in the past, such as when discussing another researcher's work" (APA, 2020, pp. 117-118).
Examples of verb tense: https://apastyle.apa.org/style-grammar-guidelines/grammar/verb-tense
References
American Psychological Association. (2020). Publication manual of the American Psychological Association (7th ed.).
American Psychological Association. (2020). Verb tense. Retrieved 26 May, 2020, from https://apastyle.apa.org/style-grammar-guidelines/grammar/verb-tense