I am not sure when to omit and when to retain the subject of an active sentence when using passive voice. The following examples illustrate my dilemma.
Can we solve this problem?
- can this problem be solved by us?
- can this problem be solved?
According to my book second sentence is the passive voice for the given statement (omitting the subject)
Have you shut the door?
- Has the door been shut by you?
- Has the door been shut?
Whereas for this question my book gave '1' as the correct passive construction (retaining the subject).
Is my book correct with both these sentences? When should we omit the subject in general? Please explain with suitable examples.