SQL Help: Multiple LEFT OUTER JOINs

I need to generate a table of teachers, and the count of classes they
taught in the past and are scheduled to teach in the future.

id | last_name | total future_class_count | past_class_count
-----+--------------+----------+--------------------+-----------------
3 | Smith | 12 | 3 | 9
8 | Jones | 0 | 0 | 0

table person
id
last_name

table class
id
class_time

table role
id
role_name -- for limiting to a type of teacher

-- link tables

table person_role
person references person
role references role

-- This table ties a person to a class, thus making them an instructor

table instructors
person references person
class references class

I can easily get instructors and the total count of their classes:

SELECT person.id AS id, last_name, count(instructors.class)

FROM person LEFT OUTER JOIN instructors ON (person.id = instructors.person),
person_role

WHERE person_role.person = person.id
AND person_role.role = 3 -- limit to this type of teacher

GROUP BY id, last_name;

Here's where I'm missing something. Trying to do an outer join on
to bring in the class row with its class_time column:

SELECT person.id AS id, last_name,
count(instructors.class) as total,
sum (CASE WHEN class_time > now() THEN 1 ELSE 0 END) as future_class_count,
sum (CASE WHEN class_time <= now() THEN 1 ELSE 0 END) as past_class_count

FROM (person LEFT OUTER JOIN instructors ON (person.id = instructors.person)) t
LEFT OUTER JOIN class on ( t.class = class.id ),
person_role

WHERE person_role.person = person.id
AND person_role.role = 3

GROUP BY person.id, last_name;

--
Bill Moseley
moseley(at)hank(dot)org