SQL Query to check Range

I am trying to write a SQL query that basically takes the starting and ending values that have already been inserted for a particular day and compares them with all of the starting and ending values to output everything that doesn't conflict. In other words, I am writing a scheduling application, that should tell a person the courses that they cannot take due to a scheduling conflict. This is what I have right now:

    SELECT C2.COURSENUM, C2.SECTIONID
    FROM TIMES T2, CLASSES C2
    WHERE T2.SECTIONID = C2.SECTIONID AND 
    (T2.MONSTART >= 
        (SELECT T.MONEND
        FROM DEGAUDIT D, UNDERGRAD S, TIMES T, CLASSES C
        WHERE D.SID = S.SID AND S.SSN = ? AND C.SECTIONID = T.SECTIONID 
        AND D.SECTIONID = C.SECTIONID)

        OR T2.MONEND <= 

            (SELECT T.MONSTART
            FROM DEGAUDIT D, UNDERGRAD S, TIMES T, CLASSES C
            WHERE D.SID = S.SID AND S.SSN = ? AND 
            C.SECTIONID = T.SECTIONID 
            AND D.SECTIONID = C.SECTIONID))

Where MonStart and MonEnd are my starting and ending times respectively. Right now, I am having troubles because both subqueries return more than one value which you cannnot use with the <= symbol or the >= symbol. Is there a way to accomplish this task using IN or NOT IN? Thank you! Your help is greatly appreciated.