Thank you very much, guys. This is exactly the kind of experienced help I'm looking for (outside the classroom). Well, I guess I can be thankful for already knowing English and its countless, strange rules and counter-rules!!! Anyway, I'm glad to hear that speaking this foreign language becomes more like your natural tongue everyday. For an example, Pucca, try this:
Yo escribo la carta a ti.
Yo te escribo la carta. Or Yo te la escribo.
"la carta" is the direct object because it receives the verb's action directly; hence, "I write what? "- 'la carta' - the letter. Then, the direct object can be replaced by the direct object pronoun "la". "a ti" is the indirect object because it answers the question to whom? or for whom?. Thus, "I write the letter to whom?" - 'a ti' - to you. Then, the indirect object must be used in conjunction with the indirect object "te". So, the first sentence is gramatically incorrect without te.
Thanks again, everyone.
Sincerely, Víctor de Michigan