How to learn basic english?

Passive voice is used when the focus is on the action. It is not important or not known, however, who or what is performing the action.

Example: My bike was stolen.

 

In the example above, the focus is on the fact that my bike was stolen. I do not know, however, who did it.

Sometimes a statement in passive is more polite than active voice, as the following example shows:

Example: A mistake was made.

 

In this case, I focus on the fact that a mistake was made, but I do not blame anyone (e.g. You have made a mistake.).