Sentences should either be completely in active voice (where the subject is doing the action) or completely in passive voice (where the subject is being acted upon).  It is best not to switch from active to passive voice or vice versa halfway through as that can make the sentence choppy.

Example:

Incorrect: Jared ate the pizza, but the cookies were taken by Lana.

Correct: Jared ate the pizza, but Lana took the cookies.

Additional resources:

“When Words Collide” Seventh Edition, pg. 81

“When Words Collide” Eighth Edition, pg. 95

“When Words Collide” Ninth Edition, pg. 81

Sometimes the shift from past to present voice is needed when showing the movement of time (see example one).  When this is not the case, keep your verb tense consistent to not confuse the reader (see example two).

Examples:

Example one
Correct: There was once a time that Becky dreamed of owning a shelter, but now that dream is gone.

Example two
Correct: David walked up the street as the motorcycle zoomed by.

Incorrect: David is walking up the street as the motorcycle zoomed by.

Additional resources:

“When Words Collide” Seventh Edition, pg. 81

“When Words Collide” Eighth Edition, pg. 96

Stay consistent in either first, second or third person as you are writing or your reader might become confused.

Examples:

Example one
Incorrect: The Joneses should start doing our projects or they could fall behind.

Correct: The Joneses should start doing their projects or they could fall behind.

Example two
Incorrect: We like doing activities by themselves.

Correct: We like doing activities by ourselves.