relative pronouns and subject-verb agreement?

[dihen]

Is it true that the third person singular shouldn't be used it the modified pronoun is the first-person singular or the second-person?

So it should be "It is I who am sad."
and "It is you who are happy.".

But would any native speakers say it like this? :

"It's me who's sad."
"It's you who's happy."

[dilekdemir89]

I think both of them are correct; one is formal while the other is less formal, but I'm not sure which one:)

[Tdol]

'It's me who's sad' is common, probably more common than 'I who am', which is formal.