Do we always use were with the 3rd person in imaginary situations?
If only she were here ...
If only she was here ...
Are we supposed to use were every time there is an if in the beginning of a sentence?
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Were is the singular Subjunctive form (I were, you were, it, etc, were).
It would be best to use this form when writing of an action or condition that is not positively so.
In current usage, the Subjunctive is not used as much as it once was. But it is, all the same, correct to use the subjunctive when writing of anything uncertain.
If only she were here...
If only she was here....
may be acceptable these days. But I think it best to be as correct as reasonably possible, so I would use the first example. It is hardly ever wrong to be correct.
The subjunctive mood is about the possibility of an event.
In English, the subjunctive mood is used to explore conditional or imaginary situations. grammarist.com/grammar/subjunctive-mood/
If only she were here ... The word 'were' is unexpected, so it alerts the reader or listener to a shift of dimension.
If only she was here ... This is incorrect English, but you could say in a different setting, "Well, if she was here earlier that would explain why she knew the menu."