sql-server - SQL Server trigger with insert, update, delete

Question

Previously, I have created 3 triggers individually for insert, update, and delete actions.

The triggers are supposed to modify a value in another table when one of these happen.

When I try to create one combined trigger for all 3 actions (insert, update, and delete), I get many errors when trying to execute it. I've tried both case statements and if statements, but cannot tell what is going wrong.

CREATE TRIGGER multipurposeCourseEnrollment
    ON courseEnrollment AFTER INSERT, DELETE, UPDATE AS
        BEGIN 
            IF EXISTS(SELECT courseID FROM DELETED) AND EXISTS(SELECT courseID FROM INSERTED)
                THEN
                    DECLARE @oldCourseID INT
                    DECLARE @newCourseID INT
                    SELECT @oldCourseID = (SELECT courseID FROM DELETED)
                    SELECT @newCourseID = (SELECT courseID FROM INSERTED)
                    BEGIN
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats +1)
                            WHERE courseID = @oldCourseID
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats -1)
                            WHERE courseID = @newCourseID AND courseOpenSeats>0
                    END
            ELSE
            IF EXISTS(SELECT courseID FROM DELETED)
                THEN
                    DECLARE @courseIDDel INT
                    SELECT @courseIDDel = (SELECT courseID FROM DELETED)
                    BEGIN
                        UPDATE courses SET courseOpenSeats = (courseOpenSeats +1)
                            WHERE courseID = @courseIDDel
                    END
            ELSE
            IF EXISTS(SELECT courseID FROM UPDATED)
                THEN
                    DECLARE @courseID INT
                    SELECT @courseID = (SELECT courseID FROM UPDATED)
                    BEGIN


        UPDATE courses SET courseOpenSeats = (courseOpenSeats - 1)
                        WHERE courseID = @courseID AND courseOpenSeats>0
                END
        ELSE
        END)
    END;

One problem may be how I declared the @variables. Not sure if this is correct.

The error I get is :

Incorrect syntax near the keyword 'THEN'.

I also get this for ELSE