Given this sentence, the that feels unnecessary:
If I believed that I were in a position to do so, I would.
I find these seemingly spurious instances of that working their way into my prose all the time.
Another example where it seems unnecessary:
I’ve been around people so critical that I ceased hearing them.
My question is: What is the grammatical basis determining where that is appropriate in sentences like these?
We can omit that in all positions, except when the that goes
at the beginning of a sentence . . . or when the that-clause comes
after an abstract noun . . . We usually omit that in speech.
‘An A-Z of English Grammar and Usage’ by Leech and others.
In relative clauses, that is usually omitted unless it is the subject.
If I were and if I was are both grammatical in British English, but not, apparently, in American English.