Some of what students are taught about English is false. One of the biggest English grammar myths is that all sentences in a story or other text should be in the same tense. When students are told to write a story the instructions on tense sound like this:
Tense tells the reader the time. What time is it in your story? Whatever time it is it should remain the same throughout your whole story. If it is yesterday you are talking about, stay there.
The problem with this is that it is not true for English outside the classroom. Teachers tell students to use only one tense, because it is a simple instruction. Many student compositions have a confusing mix of past and present.
I looked at the cake. I feel hungry.
This mixture is wrong and confusing. Has it already happened or is it happening now? Tense should answer that question, but when two tenses are used we are uncertain. A better, but more complex instruction is that all sentences in a story should use tense to show a consistent point of view in both time and location. Time is explained by tense.
Sentences in a story should use tense to show a consistent point of view.
In many stories, written by good writers, tenses get mixed. Consider these sentences:
The ground is wet because it rained yesterday.
The ground is wet because it had been raining yesterday.
The first sentence tells us two true things, the first in present tense "The ground is wet..." and the second in past tense "... it rained yesterday". Both of these true things are told in their correct tense. It is possible to use the perfect form as the second sentence shows, but it is slightly more awkward. Both are correct.
When should the writer switch tenses? There are three main times to switch tense: quotations, facts and time.
1. A quotation in an essay or story might not be in the same tense as the rest of the text. Don't change the quotation, use only what was said; quotations are fixed.
2. General facts are normally left in the present tense no matter what tense the rest of the story is told in.
Water is wet.
Reports a fact.
Water was wet.
Water will be wet.
These two sentences suggest that water is not wet now. While that might be fine in a science fiction story, if that is the plot, it should not be used elsewhere.
3. Tense should change when the time of events requires it. So events that happen at different times should be clearly indicated by tense.
Someday, billions of years from now the sun will explode.
In a story told in either the past or present tense the above sentence must be in the future tense. (Unless of course the sun has exploded in the story.) So tense should also change when the text requires it to make sense.
The tense consistency rule is taken from the classical English style text, Strunk's Elements of Style, rule 17. It is, "In summaries, keep to one tense." part of what he explains is that, "If the summary is in the present tense, antecedent action should be expressed by the perfect; if in the past, by the past perfect." A summary of an event should be clear and quick to read and should be in one tense. So most newspaper articles, encyclopedia entries and similar reports need to be in one tense. Stories and other texts just need to have tense make sense.