In the book 'Angels and Demons' by Dan Brown there is a sentence from a scene in which he is describing the media putting up their outside broadcast equipment. - 'If a screen were well situated - in front of the action for example - a competing network could not shoot the story without including an advertisement for their competitor. Is were correct here? or should it be was? or even is? or could be? or had been?
"Were" here is an example of the subjunctive mood, which, in English, is dying out. Many people now write "was" in this situation; others consider the subjunctive to be more elegant or correct, but they seem to be fighting a losing battle.
"If a screen were..." describes a hypothetical situation that hasn't actually happened yet, and the author doubts that it will happen in the near future.
It may be that the author is describing some kind of rules or laws about the placement of such screens. The law prohibits the erection of such screens because if they were carefully placed -- but they can't be because the law prohibits this so this is only a hypothetical what-if scenario -- then... (etc.)
If what he means is that, in the past, such laws existed, then he would have to use the indicative mood, and "If a screen was" would be correct (but, because the subjunctive is dying out, might be a bit ambiguous if taken -- as here -- out of context).