Grammar: verbs, some terminology
Three sets of terms:
"I ate an apple.' This is described as 'transitive' because the word that is the verb ('ate') seems to suggest that the subject ('I') is doing something (eating) to something else (the apple). This is also described as 'active' because the 'agent' (ie the person doing the doing (!) is acting directly upon the verb. As you can see these are 'horizontal' descriptions.
'The apple was eaten by me.' The verb is now being used passively, 'in the passive'. The meaning is almost the same - though you could argue the focus is different. The 'agent' ie 'I' or 'me' is still in the picture. But now the action (eating) being done is being done on 'the subject' of the sentence (the apple), the word that 'governs' the verb. (We'll come back to that when we do 'subject-verb agreement').
With the use of 'is' or 'was' phrases like in 'The apple was eaten' there's a way in which the meaning rebounds back off the verb ('was eaten') on to the subject ('the apple'). The agent in a passive sentence like this is not the subject. The agent is on the end in that little phrase 'by me'. It's 'me'.
'The apple was eaten.' Here the meaning has shifted even more. All we know is what's happened to the apple. We don't know who did the eating. It's passive without an agent.
If we were in a class together now, the fun would be to look at a newspaper or something like it and see if we could spot transitive, intransitive, passive and active constructions.
Note: these descriptions of verbs are not exhaustive. That's to say, they don't describe all ways in which verbs are used. Again, in a class, we could go off hunting for these.
How would you classify these:
'I like walking.'
'I go walking.'
'I walked the dog.'
'The dog was walked by me.'
'The dog is liked.'
Teachers: you'll see on the sample questions for the SPAG test, the children will be told to sort the sentences into active and passive and/or create some passive sentences from active (or vice versa perhaps). However, they will always give an 'illustration'. You may find that the easiest way to teach this is indeed 'by example. The constructions that they give are relatively simple and regular. This means that it's possible to get the answer right by imitation without having to work it out, understand it, or get it from first principles. There is absolutely nothing wrong with getting grammar questions right by imitating things! Means to an end, eh?
I don't think that the children will have to deal with more complicated sentences like:
'The car will have been driven'
'The car would have been driven'
'The car might have been being driven'
At least, we can hope not, can't we?
Now 'expletive sentences':
'There's a chair over there.'
'It's a chair.'
'There are 45 peas on my plate.'
These are of course amongst the most common kind of sentences in English, though the more you look at them, the more subtle differences you start to find.
'THERE'S the chair' seems to be different from 'There's the chair'. The first 'there' seems to be more 'demonstrative' than the second.
Can an expletive sentence be passive?
'It is thought that people who murder should be hanged.'
'It is felt that poor people should be put in prison.'
These and some related expressions are an ideal resource for historians and politicians wanting to claim that everyone (or important people) have a particular view on something.
'There is a view that...'
'It is normal to think that...'
'There is a general sense that...'
These aren't passive grammatically but have a similar sense to the passive expletive constructions.
If you want some fun, listen to politicians being interviewed and see how many times, they claim that a view is just generally held by an invisible 'we' or 'us'. When they do, they find these passive expletives very useful.