Use one tense consistently unless you have a reason for changing. Tenses can be troublesome in academic writing. Follow the conventions for the discipline in which you are writing.
*Strategy: Underline all the verbs in your rough draft; then go back and check for consistency of tenses.
discuss texts in present tense.
Shakespeare uses figurative language in his sonnets.
This historian flawlessly argues her point about the Clinton administration.
but use past tense to refer to past events.
Shakespeare was born in 1564.
The Clinton administration caused controversy with its health care plan.
often describe their experiments in past tense and their results in present tense.