Tense, the time in which you place the action, typically needs to be kept consistent within a single sentence or paragraph. Tense needs to be kept in a logical relationship. If you shift tenses, ask yourself whether the shift is justified.  Here are several potential trouble spots:

An author's assertion

The author's statement is introduced by the on-going present tense. The author wrote the statement only once in the past; however, because the assertion is in print, it continues to make that same statement to each successive reader.  It is ongoing in the present, in other words.

George Orwell warns in 1984 that in the future, Big Brother will be watching.

Kennedy's words, "Ask not what your country can do for you," ring true today.

For similar reasons, the present tense should be used when discussing literary figures and events.

Huck learns about social hypocrisy as he travels down the Mississippi River with Jim.

Past events

When you are writing about an event or concept completed in the past, express it in past tense.

Orwell wrote his novel 1984 in 1948.

Kennedy delivered his inaugural address in 1961.

When writing about an event that took place before the time of the events depicted in a literary work, it is appropriate to use the past or the past perfect tense (had + past participle).

The Widow Douglas had cared for Huck as a son.

Verb shifts

In the example below, the writer inadvertently shifts from present to past, confusing the reader.

Wrong:  The flowers bloom in the garden, and the trees opened up their leaves.

Some situations, however, do require a verb shift. You need to change the verb tense from past to present when someone makes a claim that continues into the present.

In the 1950's, President Eisenhower initiated the interstate highway system that is undergoing major renovation today.