Verb tense consistency is especially important in modern English. In most cases, verb tenses must remain the same within and among sentences, paragraphs, and entire passages. Yet, there are a handful of situations where a shift in verb tense is not only acceptable but necessary to help express a particular idea.
English verb tenses need to remain consistent when events expressed are directly connected:
After she left school, Marie went to the gym.
On one particular day, Marie went to the gym after she left school. One-time events take simple past tense verbs.
After she leaves school, Marie goes to the gym every day.
Marie goes to school then goes to the gym on a regular basis. This is part of her routine, or what we call a “habitual” action. Habitual actions take simple present tense verbs.
Because one action directly follows the other action, we can’t mix and match verb tenses:
After she left school, Marie goes to the gym every day. (Unacceptable shift from past to present)
After she leaves school, Marie went to the gym. (Unacceptable shift from present to past)
Occasionally, it is acceptable to shift tense in the middle of or between sentences. In these rare instances, a shift in tense is necessary to accurately express the relationship between events:
2001: A Space Odyssey is [present] a film that has confused [past] and delighted audiences since it was released [past] in 1968.
2001 was released on a specific day in 1968. While this was a one-time event, the fact that it continues to provoke strong reactions is an essential, or habitual, characteristic of the film. Audiences did, do, and (presumably) always will respond to 2001 with a combination of confusion and delight.
At the time, I thought [past] that my ESL teacher was being [past] needlessly precise about the rules of English verb tenses. However, now I appreciate [present] just how tricky these tenses can be.
In the first sentence, the speaker is reflecting on how he or she felt about his or her English teacher’s methods when he or she was studying English verbs. In the second sentence, the speaker expresses a newfound appreciation for those methods.
The speaker shifts from past to present to contrast how he or she felt then with how he or she feels now. To help clarify that he or she is contrasting the past with the present, the speaker includes the phrases “at the time” and “now.”