Grammar lesson on verb-tense shift

https://www.thoughtco.com/tense-shift-verbs-1692461

NORMAL USAGE OF VERB TENSES

[Verb] tense places the action of the verb in time: 

Today I go. Yesterday I went. Tomorrow I will go. 

Different verbs in a sentence or paragraph may logically use different tenses to reflect actions at different times.
We will play tennis before we eat breakfast but after we have had our coffee.

The tense you select to describe most of the actions in your paper is called the governing tense. Once you establish it, do not use another tense without a good reason.

Usage Tips: Avoiding Needless Tense Shifts

What is an example of an unmotivated shift in tense in writing? One example is starting a story in past tense and suddenly shifting into present tense:

Last week I was walking along a street when this man walks up to me and says . . .

“We do this in speech all the time, but in formal writing it's considered to be an error." —Edward L. Smith and Stephen A. Bernhardt, Writing At Work: Professional Writing Skills for People on the Job. NTC Publishing, 1997)

UNUSUAL USAGE OF VERB TENSES

It is common practice to analyze literary works in present tense. Thus, you would write, 'Pearl is a difficult child' rather than 'Pearl was a difficult child' in an analysis of Hawthorne's The Scarlet Letter.

It is likewise common to discuss articles and essays in present tense:

King’s article states that…

In the article, Cisneros claims that…