When can/should a subject be omitted in Passive Voice?

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?

1. can this problem be solved by us?
2. 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?

1. Has the door been shut by you?
2. 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.

• omitting the subject may lead to ambiguity for the reader. Can the problem be solved...by whom? How is the reader supposed to infer? So retaining the subject makes it unambiguous for a reader. Oct 19, 2015 at 10:10
• I am with you on this. But there are examples where the subject is omitted intentionally.
– Abhi
Oct 19, 2015 at 10:20
• Yes, it can be done where the meaning would be clear to the user and there would be no ambiguity. "All right, open your books to page 31," said the teacher to the students. The books were duly opened to page 31. (here's it's obvious who opened the books). Hence, we can safely omit it. Numerous such examples exist. This may help. Oct 19, 2015 at 10:31
• @Abhi A good book wouldn't tell you that your #2 (Has the door been shut?) is incorrect. A really, really good book may tell you why and when we may use it (along with other passive sentences in general), not just how you can transform the active voice into the passive voice, but that's rather rare. Oct 19, 2015 at 14:50

2 Answers

Often, if you need to specify the agent for a passive construction, you're probably better off using the active voice. I would prefer "Can we solve this problem?" and "Have you shut the door?" to either passive. Use the passive (without an agent) where the emphasis is on the action, not the person(s) performing the action.

• Well, yeah. But I need to make the passive constructions for the given statements. That's where I am stuck.
– Abhi
Oct 19, 2015 at 18:55
• In each case, both examples are correct. It's not right or wrong to omit the agent; it's just a matter of how much information is conveyed by the sentence. Oct 19, 2015 at 19:30

All four of your sentences are correct, and passive. You can choose to include a by-phrase or not, depending which version better suits the purpose of the sentence.

If you ask

• can this problem be solved by us?

I might reply "No, let's get someone else to work on it".

But if instead you asked

• can this problem be solved?

It would be a different question and I might answer "Yes, Paul can solve it", or "No, let's try to solve a different problem instead".