You will use many different tenses within your writing and shifts between tense are fine, as long as they accurately represent the action described by the passage. Where learners sometimes encounter difficulties is when they mix verb tenses within a sentence or description.
For example, "the tide is high and it flooded the road".
For more information, examples, and self-test exercises, please refer to The OWL at Purdue: Verb Tense Consistency.
Deciding the appropriate verb tense usually comes down to using the one that best reflects the time period of the action described in the text.
The choice of using present or past tense in signal phrases for paraphrases or quotations largely depends on the discipline in which authors are writing or the style guide they’re following.
According to APA Style, "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" (American Psychological Association [APA], 2020, p. 118). When expressing "a past action or condition that did not occur at a specific, definite time or to describe an action beginning in the past and continuing to the present", use the present perfect tense (e.g, Lee (2015) has used...).
Using the past tense to refer to other researcher's work reflects that the quotation or paraphrase presents the author’s thinking at the time of writing the text, which happened in the past. The published text may not reflect the author’s current thinking, so putting the signal phrase in present tense makes a claim that can’t be investigated within the source material. If you’re unsure of which tense to use in signal phrases, please check with your instructor, supervisor, or journal editor.
Describe Action in Text
The APA Style manual provides suggestions on which verb tense is appropriate for various sections of a thesis, major project or journal article:
As much as possible, try to be consistent with your chosen verb tense within a section "to ensure smooth expression" (APA, 2020, p. 118). If the verb tenses suggested above don't make sense for the purposes of your document, please check with your instructor or academic supervisor to get their opinion on the best approach for your document.
American Psychological Association. (2020). Publication manual of the American Psychological Association (7th ed.). https://doi.org/10.1037/0000165-000