"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?

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

In addition "separate" and "apart" are redundant. If you separate, you put something apart for other things.

In this case, the correct answer is "If residents were to separate recyclable materials from the daily trash," and not "if residents separate recyclable materials from the daily trash." The reason for this is that the use of "were to" indicates a hypothetical or conditional situation.

In the given sentence, the use of "were to" suggests that the action of separating recyclable materials from daily trash is hypothetical, emphasizing that it hasn't been implemented yet. It implies that if residents were to engage in this behavior, the cost of refuse disposal would be greatly reduced. This structure is commonly used to discuss hypothetical or unreal situations, rather than factual or straightforward statements.

On the SAT and many standardized tests, attention to grammar accuracy is essential. While your suggestion of removing "were to" might make the sentence shorter and simpler, it fails to express the appropriate conditional meaning. The correct answer adheres to standard grammar rules and effectively conveys the intended conditional message.

So, when encountering similar questions on exams, it's important to carefully consider the nuances and conditional meanings conveyed by specific sentence structures.