Generally we use both ‘it is I’ and ‘it’s me’ to introduce ourselves. The only difference is that ‘it’s me’ is more common and casual whereas ‘it is I’ is just formal, and will sound outdated. We are hearing ‘it’s me’ more often in our day-to-day conversations. Some people are of the opinion that in academic sense, ‘it is me’ is not grammatically correct, but is spoken frequently by most English speakers.
Most grammar style books suggest that when a pronoun follows a linking verb, such as ‘is,’ the pronoun should be in the subject case. It’s also called the ‘nominative.’ Now it is correct to say, ‘It is I’:
Who called Nancy? It was she.
Who told you about the accident? It was I.
Who settled the bitter argument between the passengers? It must have been they.
Who takes care of the lawn? It is we.
Now you might be wondering if all these statements are grammatically correct. Yes, they are! That’s traditional grammar and these rules are being followed for ages. During the course of the eighteenth century, the rule relating to pronouns was that – a pronoun in the nominative case (subject pronoun) must follow a form of to be:
It is I.
It is we.
It is they.
Basically, this rule is based on Latin grammar. However, the rule does not get in the way of most native English speakers who are quite comfortable speaking ‘it’s me.’
It is me. (colloquial)
It is I. (literary)
Notice that ‘it’s me’ is also idiomatic English whereas ‘it is I’ is not.
In her book ‘Woe Is I: The Grammarphobe’s Guide to Better English in Plain English,’ Patricia O’Connor writes that almost everyone says, ‘It is me,’ and that the ‘It is I’ construction just seems to be extinct:
According to Oxford Dictionary, traditional grammar teaches that it is correct to say ‘between you and me’ and incorrect to say ‘between you and I.’
In telephonic conversation, when someone asks, ‘Is Frank there?’ Frank’s response might be, ‘This is he.’ However, in a face-to-face interaction, Frank is much more likely to say’ ‘it’s me.’
Do not assume that the usage of to be followed by a nominative pronoun (I, you, he, she, we, they) has completely vanished from our conversations or is destined to be. It’s just passing out of use in the modern language.