Here's why:
* Indirect objects receive the direct object. They answer the question "to whom?" or "for whom?" the action of the verb is performed.
* Prepositional phrases modify a noun or pronoun. They provide information about location, time, manner, or other aspects of the noun or pronoun.
Example:
* Correct: "He gave her a gift." (Her is the indirect object, receiving the gift)
* Incorrect: "He gave a gift to her." (To her is a prepositional phrase, modifying "gift")
In the incorrect example, "to her" modifies "gift" and tells us the direction of the giving. It doesn't directly receive the gift.
Key Takeaway: Indirect objects are always noun or pronoun phrases that directly receive the action of the verb, while prepositional phrases modify nouns or pronouns.