I is used as a subjective pronoun; it is used as a subject of a verb. Here 'I' becomes the subject, 'doing' the verb.
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Me is an objective pronoun and it is used for the object of the sentence. It is used for the receiver of the action of the verb.
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Me can also be used as a preposition, such as 'to' or 'with'.
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This confusion usually occurs when you have I/me connected to another pronoun or name with "and" or "or. " I believe that the confusion begins when someone says something like "John and me are ready" and that is corrected to "John and I are ready. " The speaker then thinks, "Oh, the word 'and' means that I should always use I. " This is not the case. "And" has nothing to do with it; the reason you say "John and I" in that sentence is that "John and I" are the subject. If they were the object, you would use me: "He told John and me to get ready. " If you are not good with grammar concepts like subject and objects, there is still a very easy way to decide whether to use I or me: try out the sentence with just I or me (or if you need a plural, we or us - "we" is equivalent to "I" and "us" is equivalent to "me. ")
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