Most Spanish verbs are regular in the preterite tense. The Spanish preterite tense is used to describe completed actions in the past. For example: Fui al cine ayer. - I went to the cinema yesterday.
Normally in the past tense, you do not need to worry about subject-verb agreement because you can conjugate most regular past tense verbs to the singular or plural by adding an -ed to the end of the ...
One’s tense will vary depending on what one is trying to convey within their paper or section of their paper. For example, the tense may change between the methods section and the discussion section.