Tú le dices a la profesora que el examen fue difícil.
The <u>indirect object</u> replaces the one who receives the verb. It can be replaced, if it is in the third person, "le" (singular) or "les" (plural); they can also be replaced by unstressed pronouns (me, se, te, nos, os).
In this case, as it is "<u>la profesora</u>" (the teacher), the stressed pronoun is "ella" (she), that is, it is replaced by "le".
<h3><em><u>MissSpanish</u></em></h3>