Here's why:
* Subject: The subject of a sentence is the person or thing performing the action of the verb. It answers the question "who?" or "what?".
* Indirect Object: An indirect object receives the direct object. It answers the question "to whom?" or "for whom?".
Let's look at an example:
Sentence: John gave Mary the book.
* Subject: John (He is the one performing the action of giving)
* Direct Object: the book (This is the thing being given)
* Indirect Object: Mary (She is the one receiving the book)
As you can see, the subject "John" cannot also be the indirect object "Mary" because they are performing different roles within the sentence.