What are those "if-clauses" in which would is not acceptable?In certain clauses, should is used for all three persons:
Would is not acceptable in these if clauses, but it does appear in other kinds of conditional clauses:If I/you/he should decide to go, we will need a larger car. If it should begin to snow, we will stay here tonight.
The American Heritage Dictionary entry: shouldHe might surprise you if you would give him a chance
What are those "other kinds of conditional clauses" in which would is acceptable?