Errors in subject-verb agreement have a variety of causes, most of which involve words which separate the subject from the verb. The traditional approach has not been able to deal with such errors because the typical instruction is to give students sets of sentences in which the subject and verb are both next to each other and easy to identify. In the decade plus that I have been working with students on grammar, I have often seen students correct such errors, without prompting, as soon as they identified the subject and verb in a sentence. The problem, in other words, is not that students cannot remember or understand that "a subject must agree with its verb in number." Rather, it is that the students cannot identify the subjects and verbs. The KISS approach, of course, forces them to do so.