Answer:
Writing often involves telling stories. Sometimes we narrate a story as our main purpose in writing; sometimes we include brief anecdotes or hypothetical scenarios as illustrations or reference points in an essay.
Even an essay that does not explicitly tell a story involves implied time frames for the actions discussed and states described. Changes in verb tense help readers understand the temporal relationships among various narrated events. But unnecessary or inconsistent shifts in tense can cause confusion.
Generally, writers maintain one tense for the main discourse and indicate changes in time frame by changing tense relative to that primary tense, which is usually either simple past or simple present. Even apparently non-narrative writing should employ verb tenses consistently and clearly.
General guideline: Do not shift from one tense to another if the time frame for each action or state is the same.
Examples:
1. The instructor explains the diagram to students who asked questions during the lecture.
Explains is present tense, referring to a current state; asked is past, but should be present ( ask ) because the students are currently continuing to ask questions during the lecture period.
CORRECTED: The instructor explains the diagram to students who ask questions during the lecture.
2. About noon the sky darkened, a breeze sprang up, and a low rumble announces the approaching storm.
