What does the "も” mean in "ーてもいいですか”

Question

When asking for permission to do something in Japanese, a common form is the te form verb + もいいですか . My question is does the te form "nounify" or nominalize the verb and what grammatical purpose does the も serve in this construction.

Answer 1

も means "also".

As for the difference between 家に来ていいよ and 家に来てもいいよ, 家に来ていいよ feels like the speaker knows that the listener wants to go to his house, 家に来てもいいよ feels like the speaker doesn't know if the listener wants to go to his house.

As for the difference between 家に行っていいですか？ and 家に行ってもいいですか？, they have less difference than 家に来ていいよ and 家に来てもいいよ.

But the former one feels like the speaker requests the permission more strongly or the speaker knows that the listener may permit because he always goes there.

The latter one feels like the speaker doesn't know if he would permit.

Answer 2

As you have indirectly mentioned, this can be considered as a construction of て + も.

［接助］《接続助詞「て」＋係助詞「も」から》動詞・形容詞と一部の助動詞の連用形に付く。ガ・ナ・バ・マ行の五段活用動詞に付く場合は「でも」となる。

There are several definitions listed under this entry, but I think the first definition is the most appropriate:

未成立の事柄を仮定条件として述べ、その条件から考えられる順当な結果と対立する内容の文へ結びつける意を表す。たとえ…したとしても。「失敗してもあきらめはしない」「煮ても焼いても食えない」

Translated to English, ても "expresses the intent to state a hypothetical matter that is not yet established and connects it to a sentence that is in conflict with the possible sequential consequences of that condition."

Now, for the function of も: I think も can be dropped especially in casual conversations without affecting meaning. 大辞林 第三版 says this:

「…てもいい」「…てもかまわない」など、許容を表す言い方に多く用いられて）「て」を強める意を表す。

In other words, も simply serves to emphasize て.