Erlang, just for fun:

  grade(#student{work=Work, tried_hard=Tried}) -> grade1(Work, Tried).
  grade1(excellent_work, _) -> 'a+';
  grade1(okay_stuff, yes) -> b;
  grade1(okay_stuff, _) -> 'b-';
  grade1(_, _) -> c.

Edit: formatting.

Or, if we're going to go with Prolog-inspired syntax, then why not Prolog?

    grade(Student, Grade) :-
        worked(Student, Work), work_grade(Work, Student, Grade).
    work_grade('Excellent', _, 'A+').
    work_grade('Okay', Student, 'B') :- tried_hard(Student).
    work_grade('Okay', _, 'B-').
    work_grade(_, _, 'C').

I'm a bit rusty, and I don't have a Prolog interpreter to hand to test, so I might have got some syntax wrong.

I'm always amazed that I hardly ever program in Lisp and yet these short snippets are more readable than any other language.

Yes, I've never understood why

    dostuff(x)

is considered so much more readable than

    (dostuff x)