What is the Passive?
In English sentences are either ‘active’ or ‘passive.’ E.g.
Active: Germany invaded Poland in 1939.
Passive: Poland was invaded by Germany in 1939.
In an active sentence the person or the thing responsible for the action comes first. In a passive sentence the act or the thing being acted on comes first and the person thing doing the acting comes at the end of the sentence usually with the preposition ‘by’.
In the passive version, we can even leave out the person or actor entirely.
E.G. Poland was invaded in 1939.
When do I use it?
There are a number ways we can use the passive.
When we don’t know who did the action:
The victim was shot from behind
When it doesn’t really matter who did the action:
A new HIV vaccine will be trialed next year (we don’t really need to know by who)
When you talk about things that are always true:
Aircraft are designed to be safe
When you want to emphasize the thing being acted on; when it’s you main topic:
Relativity was first proposed by Einstein in 1915; since then it has been tested to be correct many, many times.
You are writing in a scientific genre that traditionally relies on passive voice:
The compound was dissolved in water then boiled with a Bunsen burner.
Are there times when it’s a bad idea to use the passive?
Yes, sometimes passive sentences can seem a bit vague and non-specific. Also, passive sentences tend be quite long and take up more room than active sentences, so in the exam it may be quicker to write in the active.
Additionally, if you are trying to discuss the difference between two people’s ideas, passive may be confusing.