Verb Shift and Tense Errors
You need to learn three ideas to keep your tenses straight. The first is that you should be consistent in your use of tenses. Don't switch from one tense to another without reason. That holds true both within single sentences and between sentences. The second idea is that when writing about two events in a sentence, you must make sure that the sequence of events is clear. If one action happened before the other, you must adjust the tenses of each action's verb. The third is that true statements are always expressed in the present tense. This holds true for scientific truths - "Mt. Everest is the world's tallest peak." - as well as literary truths - "Cassius is jealous of Caesar." In English essays this last rule is frequently forgotten. Remember: Odysseus is always waiting on Kalypso's island, and Huck is always floating down the Mississippi.
I. Keeping Tenses Consistent
Sometimes the shift error happens within a single sentence:
Fred told me that he found out that Ethel helps at her father's store, but I think that Fred knows that since they started going together.
But more common is the unnecessary shift between sentences of a paragraph:
Jane was ostracized by the rest of the team members. They believe that her behavior was an embarrassment to the entire team.
The shift from past to present is confusing - especially since the team must have come to some judgement before ostracizing Jane.
II. Coordinating Past Actions
If one of two past actions happened before the other, make that clear by casting the earlier action in the past perfect tense:
The students realized the fruit was spoiled after they had purchased twelve crates of grapes. If they had inspected the grapes at all, they would have seen the grapes' condition. Having struggled to get the grapes home, they were frustrated with their efforts. Now, they are embarrassed to have made so foolish an error.
In every sentence of the above (dorky) little story, there are two actions: the purchasing and the realization. Because the whole point is that the purchasing takes place before the students' realization, each sentence carefully adjusts the verbs to make that action that took place during their purchase in a tense further in the past than their realization.
III. Keeping Universal Truths in the Present Tense
A universal truth is always true. That's part of its being universal. We express this by casting such statements in the present tense: "The earth's gravity hold us on the planet's surface." To put such a statement into the past sets us adrift among the heavenly bodies. The same rule applies when writing about literature. Huck is always floating down the Mississippi, and Ahab is always chasing the white whale. Of course, there will be times when writing about the sequence of events in a book that you will have to use a tense other than the present, but keep your base tense the present tense: "It turns out to be true that Gatsby had been an officer during the war."