Page 419 - Practical English Usage 3ed - Michael Swan, Oxford

413
Mistakes like these are not surprising, because:
1. Be is used to make both passive verb fOnTIS and active progressive tenses.
2. Past participles are used to make both passive verb fOnTIS and active perfect tenses. Compare:
He was calling. (active - past progressive) He was called. (passive - simple past) He has called. (active - present perfect)
For active verb forms. see 10.
passives (2): agent
In a passive clause. we usually use by to introduce the agent - the person or thing that does the action. or that causes what happens. (Note, however. that agents are mentioned in only about 20 per cent of passive clauses.)
All the trouble was caused by your mother.
These carpets are made by children who work twelve hours a day.
Some past participles can be more like adjectives than verbs (see 410): for example shocked. worried. frightened. After these. we often use other prepositions instead of by.
I was shocked at/by your attitude.
We were worried about/by her silence.
Are you frightened o f spiders?
With is used when we talk about an instrument which is used by an agent to do an action (see 119).
He was shot (by the policeman) with a rifle.
passives (3):
When do we use passive structures?
interest in the action
We often choose passive structures when we want to talk about an action. but are not so interested in saying who or what does/did it. Passives without 'agents' (see 413) are common in academic and scientific writing for this reason.
Those pyramids were built around 400 AD.
Too many booles have been written about the Second World War. The results have not yet been analysed.
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passives (3): When do we use passive structures? 414
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