I'm preparing for my exam and need to know how to convert sentences from active to passive. My first response to this question was Slowly, he walks on the road. But I thought that that's still active and also sounds a bit like Yoda. While giving the answer please give another example of conversion of an active simple present tense sentence to passive.
An intransitive verb like walk cannot sustain ordinary passivization because it has no Object which can be cast as the Subject of a passive clause.
However: if the active utterance contains a preposition phrase as predicative complement, the object of the preposition can in some circumstances be cast as Subject.
Somebody slept in this bed. → This bed has been slept in.
But this works semantically only if the action of the verb has some evident effect on the object of the preposition. The example above, for instance, makes sense because a viewer might deduce the sleeping from some disarrangement of the bedclothes.
But what about this?
The road is being slowly walked on.
It's difficult to conceive circumstances in which that would be a sensible utterance—perhaps a Dungeons and Dragons game in which the dungeonmaster describes visible effects (such as footprints) of an invisible character walking on the road?
As exposed by StoneyB, passive structures are impossible with intransitive verbs. Most of these verbs include the stative verbs. Examples of these are fit, have, lack, resemble, suit.
Michael Swan — Practical English Usage (thanks to Damkerng T.) explains that some transitive verbs, too, are seldom used in the passive. Most of these are 'stative verbs' (verbs which refer to states, not actions).
Here are some examples:
» They have a nice house. (NOT
A nice house is had by them.)
» My shoes don't fit me. (NOT
I'm not fitted by my shoes.)
» Sylvia resembles a Greek goddess. (NOT
A Greek goddess is resembled by Sylvia.)
» Your mother lacks tact. (NOT
Tact is lacked by your mother.)
» She was having a bath. (NOT
A bath was being had by her.)