When to use the relative pronoun who (subjective case: who goes with he and they).

When to use the relative pronoun whom (objective case: whom goes with him and them).

       E.g. Sylvester, (who/whom?) is afraid of the dark, sleeps with a Donald Duck night-light on.

* Look only at relative pronoun in its clause. Ignore the rest of the sentence.

       Who/whom is afraid of the dark?

Answer the question with an ordinary personal pronoun. Such as “He is”.

If you answer the question with a subjective case pronoun (as you have here), you need the subjective case “who” in the relative clause.  

       Sylvester, who is afraid of the dark, sleeps with a Donald Duck night-light on.

If you answer the question with a objective case pronoun, you need the objective case whom in the relative clause.  

*Try answering the question with he/him. Who goes with he (subjective case) and whom goes with him (objective case).