Scientific writing often stresses objectivity through use of passive voice in communication of experimental data, and there are some other appropriate uses of passive voice constructions. In general, though, consistent use of passive voice can produce a lifeless, wordy text.
The kids ate the pizza.
The committee has discussed the problem.
The pizza was eaten by the kids.
The problem has been discussed by the committee.
*Strategy: To change passive voice to active voice, ask yourself, “Who/What is doing the action here?” (Answer: the kids, the committee) Then, make the actor(s) the subject in the new sentence, keeping tense the same.