Many sentences are presented in the SOVC (subject-object-verb-complement) format. The boy threw the ball. A person takes action and moves an object. When we read these sentences we know who took the action. These sentences are in the active voice. In most of our writing we want the reader to know who did what.
In passive construction, the sentence usually starts with the object, then the verb. Ronald Reagan famously said "Mistakes were made." He didn't say who made the mistakes. You might say to your parents "The car was in an accident," when you could have said "I rear-ended a police car with your BMW." See how much nicer it sounds when you leave out the subject? But in an essay we want to know who did what to whom.
Because of this, sentences in which the person doing the action is not clear are bad style for essay writing. When you write "It was discovered that America lay between Europe and Asia on the western route," you don't let the reader know an important piece of information: who did the discovering. A much better sentence would be "Columbus discovered that America lay between Europe and Asia on the western route."
Let’s see how this works.
Sometimes you will accidentally slip into the passive voice because all you want to introduce into your essay is a fact and you might not know the source without going and doing a lot more research and it's already midnight and the essay is due at 8 am. You write "It was found that operant responses were most effective in modifying student behavior." What can you do to fix this without doing more research? You could write "Student behavior is most effectively modified using operant responses." If the marker is sharp, he/she will want a reference here. However, you have avoided the dreaded passive voice.
In science writing, however, the passive voice is preferred. Why? Because scientists like to live in a fictional world where facts are facts and don't rely on human agency. So in your science report you should write "The chemicals were mixed," not "We mixed the chemicals."
When you are writing about a scientific discovery, use the active voice. Einstein proposed the theory of relativity. Newton formulated the theory of gravity.
To proofread for the passive voice look for the words "by" or "it was." These are clues that you might have written something using the passive voice.