(1)

SELECT DISTINCT sname
FROM Student, Faculty, Class, Enrolled
WHERE
    level = 'JR'
AND fname = 'I.Teach'
AND Student.snum = Enrolled.snum
AND Enrolled.cname = Class.cname
AND Class.fid = Faculty.fid

(2)

SELECT MAX(age) AS mage
FROM Student
WHERE major = 'History'
OR Student.snum IN (
                    SELECT Enrolled.snum
                    FROM Faculty, Class, Enrolled
                    WHERE
                        fname = 'I.Teach'
                    AND Enrolled.cname = Class.cname
                    AND Class.fid = Faculty.fid                     
                   )

(3)

SELECT name
FROM Class
WHERE room = 'R128'
   OR name IN ( SELECT cname
                FROM Enrolled
                GROUP BY cname
                HAVING COUNT(*)>4 )