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If I ask, "Can I borrow a pencil?" I might say: エンピツを借りてもいいですか?

Why do we use the も particle here, since we're not saying also this or that? To express also is the main usage of mo I'm familiar with.

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up vote 5 down vote accepted

Of course, 〜てもいい is not limited to borrowing, but rather any form of permission.

Being very literal...

エンピツを借りる "to borrow a pencil"
エンピツを借りていい? "Is it okay if I borrow a pencil?"
エンピツを借りていい? "Is it okay even if I borrow a pencil?"

It is certainly not ungrammatical to have a も there (syntactically, you can insert any 係助詞{かかりじょし} between the て and the following verb/adjective), and semantically it makes sense as well.

The fact that the form with the も is more popular, however, is just idiomatic.

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I was under the impression that -ていい was -てもいい with も dropped. Am I wrong? – Sjiveru Jul 7 '13 at 19:17
@Sjiveru Historically, I think 〜てもいい is 〜ていい with も inserted. However, you are probably correct that these days it's the other way around. – Darius Jahandarie Jul 7 '13 at 19:28

Okay, so looked this up, and it looks like this is an idiomatic expression of sorts as it doesn't seem to strictly follow the typical usage of the も particle. Typically も does indicate "also" something.

Yet, something + もいいですか? seems to be the idiomatic way to say, "Can I do something"

Likewise, something + もいいです。 would be the way to say, "You can do something."

Correct me if I'm wrong to consider this somewhat of an idiomatic structure, and if there actually is a grammar rule that is being used here.

Reference: http://nihongo.anthonet.com/mo-ii-desu-wa-ikemasen/

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It's mildly idiomatic, but really it's a pretty normal, if conventionalised, use of -ても. You could literally translate -てもいいですか as 'is it okay (even) if I...'. – Sjiveru Jul 7 '13 at 17:49

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