The plate, which had been eaten off, was owned by my aunt. Passives
A passive sentence in English is not an automatic transformation of an active one. It is an alternative realisation of the relation between a transitive verb and its object, and involves an intransitive construction with a subject (corresponding to the transitive object), a copula-like verb be or get, and a participial form of the verb. Thus, corresponding to A man took away the mad dog we get the passive The mad dog was taken away. A passive clause may include an NP, introduced with by, corresponding to the transitive subject (e.g. by a man), but it does so only relatively seldom (in formal, written English, more than 80 per cent of passives are agentless, and the figure is undoubtedly higher for colloquial, spoken styles).
There is always a meaning difference between active and passive constructions. There are some transitive verbs which--for semantic or other reasons--never occur in the passive (§§9.2, 9.3) and, for many verbs, ability to passivise depends on the nature of the object. As Bolinger ( 1977a: 10) puts it: 'We can say George turned the pages or The pages were turned by George; something happens to the pages in the process. But when we say George turned the corner we cannot say *The corner was turned by George --the corner is not affected, it is only where George was at the time. On the other hand, if one were speaking of some kind of marathon or race or game in which a particular corner is thought of as an objective to be taken, then one might say That corner hasn't been turned yet. I can say The stranger approached me or I was approached by the stranger because I am thinking of how his approach may affect me--perhaps he is a panhandler. But if a