public static String errorOf(Clause c) {
        String error = "subject and verb agreement error";

        ArrayList<String> subjects = c.getSubjects();
        ArrayList<String> subjectIdentifiers = c.getSubjectIdentifiers();
        ArrayList<String> actionIdentifiers = c.getActionIdentifiers();

        for (int j = 0; j < actionIdentifiers.size(); j++) {
            if (pastTenseAction(actionIdentifiers.get(j))) {
                return "";
            }
        }
        int i = 0, index;
        if (subjectIdentifiers.size() < actionIdentifiers.size()) {
            index = subjectIdentifiers.size();
        } else {
            index = actionIdentifiers.size();
        }
        while (i < index) {

            if ((singularSubject(subjects.get(i), subjectIdentifiers.get(i)) == singularAction(actionIdentifiers
                    .get(i)))) {
                return "";

            }
            i++;
        }

        return error;

    }