I want to while writing formal documents like reports etc. How to decide when to use which form of the sentence.
closed as primarily opinion-based by FumbleFingers, Drew, user140086, tchrist♦, Chenmunka Nov 16 '15 at 13:33
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Active voice is generally easier to understand, and it makes clear who/what is doing what to whom/what.
A general rule of thumb is to use passive voice only when using the active voice makes little sense or is unclear. In such cases there is often no clear subject or no subject that is significant or interesting.
This rule of thumb applies equally well to "formal" writing - reports, articles, etc. As @FumbleFingers mentioned in a comment, too many people use passive voice too often in such contexts - perhaps out of a misguided desire to seem more authoritative or more professional.
A reason why passive voice can make sense in such contexts is that the subject is sometimes not significant or interesting. But even when this might appear to be the case, it can sometimes make things clearer to name the subject: "our research team", "[Toto et al]", "we".
Often it is clear who (e.g., which research team) performed the action, however, and sometimes passive voice can be clear. Nevertheless, it is good to get in the habit of questioning your use of passive voice, just to see whether what you want to say might be clearer if you specify who/what is doing what to whom/what.