Q. Why can I say ‘My bag was stolen’, but ‘I was stolen my bag’ is wrong? (Joao, Brazil)
A good question Joao, and a common problem for many people learning English. Luckily, there is a simple explanation.

You steal something fromsomeone but you can’t steal someone!  So we may ‘Somebody stole my bag’ or ‘My bag was stolen’. A person can be robbed (ie: have their property taken from them) but not stolen (taken away by the thief), so we can say ‘I was robbed’ but not ‘I was stolen’.  This is one reason why ‘I was stolen my bag’ is wrong.

Let’s start with your first sentence, ‘My bag was stolen’. In this sentence, we are interested in what happened to your bag. If we think about it, a bag cannot do anything, somebody must do something to your bag. We can say ‘Somebody stole my bag’ if we want to talk about a person’s actions, but if we don’t know who stole the bag, then telling people that ‘somebody’ stole it doesn’t help much. In English, when the person doing the action is not known, we often use what is called the passive voice.

So how do we form the passive? And why do we use it?

We use a form of the verb ‘to be’ and the past participle of a verb, and we use the passive for a number of reasons. Let’s look at these sentences.

‘Coffee is grown in Brazil’ (Present simple passive)
Here, who grows the coffee is not important (farmers), because we are interested in the coffee, not the people who do the action. (Compare this with ‘Farmers grow coffee in Brazil’)

‘My car was stolen yesterday’ (Past simple passive)
Here, the person who stole my car is unknown. If I tell the police that ‘somebody stole my car’, will it help them to catch the criminal? Probably not.

‘A man has been arrested for murder’ (Present perfect simple passive)
In this case, the people doing the action are obvious. Who arrests people? The police do. Can any other people arrest someone? No, they can’t. So we don’t need to say ‘the police have arrested a man’ because we know that only the police can do this.

Sometimes, when we use the passive, we do want to say who did the action, but we are still interested in what happened to something, not what somebody does. If you want to say who did the action, we use ‘by’.
‘The 2008 World Cup was won by Spain’ – in this sentence we are interested in the World Cup.
But if we are interested in Spain, and what they did, we can say ‘Spain won the 2008 World Cup’.

Now, try to do these examples. Are the verb forms active or passive? Are we interested in what people and things do, or in what happens to people and things?

1. BMW cars …………………… (make) in Germany
2. I usually …………………….. (make) mistakes when I do my homework
3. Leonardo Da Vinci ………………….. (paint) the ‘Mona Lisa’
4. The ‘Mona Lisa’ ………………………………. (paint) by Leonardo Da Vinci
5. An earthquake …………………………. (kill) 10 people
6. 10 people ………………………… (kill) by an earthquake