Writers should keep the elements in a sentence consistent, avoiding any unnecessary changes in tense, voice, mood, person, number, and discourse.
Different Types of "Shifts"
Tense Shift
She caught the ball and then throws it to the catcher for the out.
Mood Shift
Take two aspirins, and then you should call me in the morning.
Point of View Shift
If someone wants to play games, you should follow the rules.
When a person... they...
Direct and Indirect Quotation Shift:
Alex asked if the potatoes were ready and can I cut them for a salad?
Sentence Construction Shift
Proctoring exams is where you watch for cheating.
Voice Shift
When the children turned on the TV, a buzzing sound was heard.
Proctoring exams means watching for cheating.
One who proctors exams watches for cheating.