When to use “I” and when to use “me”

What will be your answer if you knock on a closed door and the person inside the room asks you, “May I know who that is?” Will you say, “It is me” or will you say “It is I”?

If your answer is “It is me”, then grammatically it is wrong. The correct usage is “It is I”.

But grammarians will probably forgive you because almost everyone says, “It is me”, so much so that it has generally been accepted as correct.

Then when to use “I” and when to use “me?

“I” is first person subject pronoun, which means that it refers to the person who is performing the action of a verb.

I can speak Tamil.

In the above example, “I” is the person who is performing the action of the verb “speak” or “I” is the subject.

Similarly, “You and I can speak Tamil.”

Here, “you and I” are the persons who are performing the action of the verb “speak”.

So whenever you are the subject of an action, you should use “I”.

“Me” is first person object pronoun, which means that it refers to the person that the action of a verb is being done to.

Anand told me to leave.

Here “me” is the person on whom the action of the verb “told” is being acted on or “me” is the object of the verb (whereas “Anand” is the subject).

Anand told Arjun and me to leave.

Here, “Arjun and me” are the persons on whom the action of the verb “told” is being acted on.

So whenever you are the object of an action, you should use “me”.

Confusion

Generally confusion comes when you have “I” or “me” connected to another name or a pronoun as in above mentioned sentences,

“You and I can speak Tamil.”

OR

Anand told Arjun and me to leave.

Whenever you are confused on deciding which one to use, you remove the other person from the sentence and then try to make the sentence. When that is the case you are less likely to make the mistake of using

Me can speak Tamil OR Anand told I to leave.

Those sentences really sound wrong, don’t they?

Alternate Method

One of this writer's friends, Raji Stephen offered an alternate method. The following is how he explained it. 

Take the following sentences.

That was I who called you yesterday.
That was me whom you called yesterday.

In order to check if your usage is right or not, you can simply take out the "action" part and make a question by adding "who" (e.g., who called?, who can speak?). If the answer points to self or a group including self, then the usage should be "I" and if the answer points to another person or a group excluding you, then the usage should be "me".  

Hope this helps.