It is important to :
1) Read the entire essay first and mark the verbs.
2)Keep in mind in which time period the topic you are writing about is (present,past,future).
3)Make sure you have the coorect form of the paste tense verbs.
4)Make sure that the verbs agree with their subjects (subject verb agreement).
5)If you are not sure of a verb's tense then make sure you review a reliable resource,Wikipedia for example.
Good Luck :)