The issue here is indicated by the verb "were" in a conditional clause.
If ... then ... (and "then" might be implied rather than written) is setting up a conditional clause. That is, the "If" clause is not a fact; it's a condition upon which the main clause would be true.
http://grammar.ccc.commnet.edu/grammar/conditional2.htm
"If residents were to separate recyclable materials apart from the daily trash, the cost of refuse disposal would be greatly reduced." I said change b should be made: separate recyclable materials from the daily trash so it reads "if residents separate recyclable materaials from the daily trash, the cost of refuse disposal would be greatly reduced." But it's wrong according to my SAT book. The correct answer is "If residents were to separate recyclable materials from the daily trash." Why isn't my way correct since it's shorter and simpler?
2 answers
In addition "separate" and "apart" are redundant. If you separate, you put something apart for other things.