Verb Tense

Readers are often confused by changes in verb tense from one part of an essay to another.

Generally speaking if you start writing a paragraph in the present tense, you should not switch to the past tense or future tense within that paragraph unless doing so aids the communication of your meaning. 

Shifting from one verb tense to another anywhere in your essay should only be done when your ideas or argument clearly require it.

Example of tense switch within a sentence:

Darwin traveled to the Galapagos Islands on his ship, the Beagle, where he observes many varieties of finches. (Note: switch from past tense to present tense.)