Normally prepositional and non-prepositional indirect objects are interchangeable, for example, "she gave him it" or "she gave it to him".
Is there rule that shows when this interchangeability is not acceptable?
BokehIs there rule that shows when this interchangeability is not acceptable?
There is no rule as such. This is considered a property of the individual verb.

The general topic is called Dative Alternation. There are three verb groups: Alternating, Non-Alternating to Only, and Non-Alternating Double Object Only.

1. give, lend, pass, pay, grant, hand, mail, toss, show, sell, ...
2. donate, explain, contribute, deliver, demonstrate, ...
3. ask, cost, fine, deny, save, ...

Give me the money. Give the money to me.
*Explain me the rule. Explain the rule to me.
Ask me a question. *Ask a question to me.

Okay, so while I'm learning the right way to explain this, can you also talk about the difference between pronouns and regular nouns?
Give the letter to Peter. Give Peter the letter.
Give the letter to him. Give him the letter.
Give it to Peter. *Give Peter it.
Give it to him. *Give him it.
Why can't the "it" to in the last place there? I know it can't but not how to explain it, other than to say "it doesn't work that way."
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Thanks for the explanation CJ.
GG, in BrE "give him it" is fine. The no-no is reversing the order "give it him".
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Hi GG:
I think the problem is the personal pronoun "it", the use of articles and confusion of cases. Articles (or qualifiers) are not used with pronouns. I think that the article or qualifier is required for the direct object, but not for the indirect object.
Give the dog a bone. OK
Give the dog some food. OK

Give it a bone. OK
Give him the bone. OK
Here's a bone. Give it to the dog. Give the bone to it. OK

Give the dog an / the it. ????Emotion: big smile
That's interesting. In the US, you would never say "give him it." Give it to him.
The only way "give it him" even makes sense to me is if "it" is some man-eating beast, and you point to the least useful person in your party of survivors and say "Give it him." Even so, you'd say "Give him to it" in that case..