What is the difference between a direct and an indirect object pronoun?

A direct object pronoun means the object (the person or thing that the verb is happening to) receives the action of a verb directly WITH NO PREPOSITION between the verb and the pronoun- in other words, the path between the object and the verb is direct, without the obstacle of a preposition such as "à". A direct object pronoun replaces the "who?" or "what?" in a question. For example, compare the following; "Mary writes a letter," and "Mary writes it." The "it" replaces the "what?" (in other words, "the letter").  An indirect object pronoun receives the action of a verb indirectly. It replaces the "to/for whom?" or the "to/for what?" in the sentence. The verbs that take an indirect object pronoun are always followed by a preposition, usually "à".  For example, compare the sentences, "Mary writes to her brother" and "Mary writes to him." The "to him" is the indirect object pronoun because it replaces the question "to/for whom does the verb happen?"