Hi Marina. Even native English speakers often use "I" and "me" incorrectly. "I" is used as the subject of the sentence (e.g. "I saw her smile") while "me" is used as the object of a verb (e.g. "She smiled for me"). * "No one but I saw her smile" - correct * "No one but me saw her smile" - incorrect, but you will often hear it said this way regardless

I am sorry, but you have been advised incorrectly. The correct form is "me". "But" is used as a preposition here, meaning "except for". After a preposition you use "me". You never use "I". Leave your question up a bit longer next time. ^_^

It's correct. There is the other one:Only I saw her smile.