Why Active, Not Passive?
At some point in your schooling, a teacher or three probably told you that passive voice was wrong and that active voice was good. This is not entirely true, but for reasons of style and clarity, active voice is often preferable to passive voice. It make sentences more immediate and engaging. Despite this, many people often write passively by default, as it feels more natural or academic to them. (There's a good reason for this: Many scholars aren't very good writers, even if they are, say, brilliant thinkers.)
We can imagine, for instance, a committee of well-meaning scholars yielding this rather dry pronouncement:
A large asteroid heading towards Earth has been discovered by the scientific group.
The sentence contains some pretty important information, but this sentence, because it's passive, doesn't put the important information to the forefront. It also lacks immediacy. So how do we fix this?
Identifying and Correcting Passive Voice
All sentences in passive voice contain two elements: A form of the verb 'to be' (those are: is, are, am, was, were, has been, have been, had been, will be, will have been and being) plus a past participle, which is a verb in the past tense.
In the asteroid example, the 'to be' verb is 'has been' and the past participle is 'discovered.' The direct object that has been found is the 'asteroid.' Hence, the sentence is in passive voice. Note: sentences without a direct object - that's the agent that the subject and verb affect - are never in passive voice.
Another simpler clue in this sentence that it's in passive voice is the preposition 'by.' If you see 'by', there's a strong chance that the sentence is in passive voice. However, 'A large asteroid heading towards Earth has been discovered by the scientific group' could just as easily be 'A large asteroid heading towards Earth has been discovered,' and it would still be a passive sentence without that preposition in there. So, it's a good clue but it's not the be-all-end-all of figuring out if a sentence is in the passive voice. Here's a good rule of thumb: If 'by' isn't in the sentence and you can't tell who's performing the action, the sentence is probably passive.
To avoid Armageddon without the help of Bruce Willis, place the doers of the action at the front of the sentence and remove the 'to be' verb. So the sentence becomes
The scientific group discovered a large asteroid heading towards Earth.
And the world is saved by good style.
First-Person Sentences
It's a pretty common belief that sentences in the first person, I or we, can't be passive. After all, if I'm doing it, it can't be unclear who's performing the action, right? It's easy to fall into this trap, but first person perspective is not an escape from passive voice.
For instance, 'I was stranded on the island by the storm' is passive. You're not stranding yourself on the island. The storm is performing the action of stranding you. To make it active, change it to:
The storm stranded me on the island.