Here's why:
* Indirect objects always receive something from the direct object. They answer the question "to whom?" or "for whom?" about the action of the verb.
* Direct objects are the recipients of the action. They answer the question "what?" or "whom?" about the action of the verb.
Example:
* "I gave the book to John."
* Direct object: "book" (what was given)
* Indirect object: "John" (to whom the book was given)
Without a direct object, there's nothing to be given, received, or affected by the action, making the concept of an indirect object meaningless.
However, there are situations where a sentence *appears* to have an indirect object without a direct object, but it's actually a prepositional phrase:
* "I sent a message to John."
* *"To John"* is a prepositional phrase acting as an adverbial phrase modifying the verb "sent." It tells us where or to whom the message was sent.
Let me know if you'd like more examples!