Path Expected for Join! Nhibernate Error

It means that you using an inner join in HQL works a little different than using it in SQL. In HQL you join the tables by providing the "path", which is basically the referenced property of your class.

So instead of

SELECT t.CourseId FROM Task as t INNER JOIN Courses as c ...

you need to write

// c.Taks is the IList property in your Courses class
SELECT t.CourseId FROM Courses as c INNER JOIN c.Tasks as t ...