Hi folks,
When I was in Japan, I’d often hear people say
>お待たせしました！
And sometimes
>お待たせいたしました！
I wondered where the difference came from; now I see that the first expression uses する while the second expression uses いたす.
So I thought about another case, which was お願いします vs お願いいたします. I don’t think I’ve ever heard the latter, but in any I case I parse this expression as お願い + いたします. お願い is a noun which means “request,” “favor,” or “wish,” according to Jisho. So お願いいたします makes sense as a “compound expression.”
But when I look up お待たせ or even 待たせ it doesn’t exist as a standalone word, so the same logic as in the お願いいたします case doesn’t fit here, i.e. there’s no “source” word like お願い in the case of お待たせしました・お待たせいたしました. Am I misinterpreting or misconstruing the underlying grammar?
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Edited to answer:
The grammar is spelled out in Genki L20.
>する compound verbs do not follow this^(1) pattern. Instead, they simply have the prefix ご or お, such as ご紹介する, ご案内する, ご説明する, and お電話する.
So する compound verbs are effectively special-cased.
* [お願い](https://jisho.org/word/%E3%81%8A%E9%A1%98%E3%81%84) is a する compound verb according to Jisho.
* [待たせる](https://jisho.org/search/%E5%BE%85%E3%81%9F%E3%81%9B%E3%82%8B) is a non する compound verb, and is just a “plain” Ichidan, transitive verb
^(1)this = お + conjuctive stem + する・いたす
by Altruistic-Mammoth