Our esteemed professor has in a potentially ephemeral comment above written this fine answer, which I herebelow consign to posterity and the general community pro bono publico:
Yes, it’s a base-level rule. These terms (Su, DO, IO) are called Grammatical Relations and form dependency trees with the predicate; this is a representational practice derived from logic.
Not all languages have syntax that uses these relations, though — there are many ergative structures in the world, and for them “Subject” and “Direct Object” are meaningless terms.
“Indirect Object”, on the other hand, is usually just the receiver.
I would please prefer to talk tomorrow.
This doesn't make sense because the subject and direct object of please refer to the same person. Please is used to request one person to contribute to another person's happiness or well-being.
- Would you please talk tomorrow?
- Would he please meet me tomorrow?
I would prefer to talk tomorrow.
There are no problems with this, but you seem intent on getting please in there.
I would prefer to talk tomorrow, please.
I think this is how it might sound, but the punctuation is probably wrong. I'll get to that in a moment.
Please, I would prefer to talk tomorrow.
I expect this is fine. Why here and not at the end? I believe we start getting into the explanation of the idiom expressed in that one word.
It can be helpful to see how languages related to English do this. In Latin, one would say amabo te, "I will love you." In German, one would say, Bitte, which can mean "to ingratiate". In French, one would say, s'il vous plaît, which translates to "if you please". In each case, you are asking someone else to please you, to do you a favor.
Japanese (not related) tends to use either: a word that elevates the other person and ask them to hand something down to you (kudasai); or a word that expresses your wish and politely requests them to grant it (onegai). Again, another person is involved in contributing to your own happiness.
In English, one could get flowery and say, "If it please you, I would prefer to talk tomorrow." This is why the fourth example, "Please, I would prefer to talk tomorrow," could work. English also has an equivalent to elevating the other person and asking them to hand something down to you: "I pray [of] you, I would prefer to talk tomorrow." This usage is an example only as it is rather archaic (often using "thee" instead of "you").
Best Answer
I think you need to lose the comma so that the two verbs (is and said) relate to the same subject. Once you put the comma in, you are connecting independent clauses which each need their own subject.