One of the example sentences for the いたす grammar point says “拝借してもよろしいですか”
I’m confused as to why. It appears to me that this is an example sentence for either する or てもいい, but not いたす.
I am aware that いたす and する have pretty much the exact same meaning other than politeness level.
Does that mean いたす and する both become しても in this context or something?
Is it a mistake that I should just be ignoring?
The grammar point: いたす (JLPT N4) | Bunpro
The explanation is in the “Caution” section, you might’ve overlooked that. (and no, いたす would become いたしても, like all す ending verbs).
I assume the sentence is in there so that you learn not to blindly apply いたす to everything, but learn to recognize already humble words. If you look at the example sentences, the second to last one is also similar.
Oh I see. Thank you. So basically the example sentence is NOT an example of いたす because it’s actually an example of when to use する instead?
Yes, it’s an example when it’s incorrect* to use it. 拝借いたします is what they call “double keigo”, because 拝借する is already the humble form of 借りる, so adding いたす would be redundant, and technically incorrect.
*Of course, there are cases where technically incorrect grammar is in wide use, but you should first learn the rules before trying to break them.
Forget all this. Too many layers of performative politeness. I’m just gonna learn to speak Japanese like a delinquent and call it good 
jkjk.
Thanks for the explanation. I had read the caution section but the message didn’t really sink in until you explained it. It’s good to know some words are so polite on their own they need verbs with less intense politeness.