| | Re: Many grammar questions
It occurred to me after posting this that it might not be clear why "a guitar" should be the object rather than the subject of your example sentence : why not A guitar was the instrument used...?
The answer lies not in grammar but in the discourse organisation of English, which generally follows a "given-new" organisation of information. So eg if the first sentence was Where was John? both speaker and listener know they're talking about John - that information is "given" for the reply. The "new" information in the reply will be the location. So the sentence will be ordered : He (ie John, given) is at home (new) and not At home is John.
You question was What was the instrument used in the play? Therefore The instrument used in the play is the "given" information and will natuirally come first in the sentence - ie in subject position - with the "new" information (the answer - a guitar) at the end - in object position. The instrument used... was a guitar / It was a guitar.
Hope that's clear.