When I teach English at the English Language Centre, in Cape Town, I experience over and over again that students are not clear about WHY we use passive constructions.
In Listening and Reading, the receptive skills, students fail to understand the passive construction. They don’t recognise it even though we know that they have studied it. Most common, I think, is that the past participle has the same form as the past simple (never mind that there is an auxiliary verb there!). Also, in reading, is it perhaps because the student expects certain information and interprets it as such? For example, ‘Man shot by woman’. Perhaps the student expects to hear that the MAN shoots the WOMAN, not the woman shoots the man? In listening, the auxiliary verb can be pronounced weakly, (i.e. it is not CLEAR to the listener) and, the student hears, for example, Man (was) shot (by a) woman, in other words, man shot woman.
In the Speaking and Writing skills, there are a number of problems. Mostly I find that students just don’t use it even if they have mastered the form. Why is this? I think it is because of lack of clarity about when to use it as very often the passive is taught as an alternative to the active voice, rather than having a genuine reason for using it, for example, showing new and important information (whereas you can use INTONATION in speaking), or in formal styles, or, in my personal favourite, to avoid blame (The window got broken, Mum). Absolute clarity is required by the teacher before teaching!
I often use authentic materials to introduce and explore the passive constructions and use these to help students work out WHY it is being used, rather than presenting it as an alternative to the active. How do you teach the passive?