Passive modal structures: It can be done tomorrow.

Modal structures can be passivised.

• We can do it tomorrow. → It can be done tomorrow.
• They may close the hospital. → The hospital may be closed.

An interesting, rather complicated point about modal verbs is that their meaning usually ‘spreads over’ a whole clause. This means that one can change a modal structure from active to passive, for example, without affecting the meaning very much. Compare:

• A child could understand his theory.
  His theory could be understood by a child.
• You shouldn’t put adverbs between the verb and the object.
  Adverbs shouldn’t be put between the verb and the object.
• Dogs may chase cats.
  Cats may get chased by dogs.

With most other verbs that are followed by infinitives, their meaning is attached to the subject, so that a change from active to passive changes the sense of the sentence completely. Compare:

• Dogs like to chase cats.
  Cats like to be chased by dogs. (different and – of course – untrue)
• Dan wants to phone Anna.
  Anna wants to be phoned by Dan. (not the same meaning)