Incorrect usage of these terms is one of the most common errors native English speakers make.
They are definitely not interchangeable.
For a newcomer to the English language, the rule can be summarized like this:
- "I" is a subject. The subject is the person/thing that is doing something.
- "Me" is an object. The object is the one being affected by or related to the subject's action. Again, if the verb is "see," the object is the person/thing that the subject "sees," "is seeing," "saw," "will see," etc.
Consider these examples:
- "I see you." "I" is the subject and "you" is the object.
- "You see me." "You" is the subject and "me" is the object.
This is a gross oversimplification, but it's a solid starting point.
Using this same logic, let's assume you are asked the question, "Who did that?" Refer to my rule above, where I said, "The subject is the one that is doing something." The correct answer would be "I," "I did," or "I did that." In colloquial speaking, however, a majority of native English speakers would likely respond "me," but ironically, the same people would never say "me did" or "me did that." The easiest way to speak grammatically correctly and sound completely natural is to simply include the verb in your response: "I did."
Also, keep in mind that "I" and "me" always come last in a series of nouns and/or pronouns. "You and I went together" is correct, while "I and you went together" is not. Some examples:
- "John, Maria, and I studied together last night." "I" must come last in the list of subjects who studied together.
- "The professor helped John, Maria, and me study last night." "Me" must come last in the list of objects who were helped by the professor.
It's just a hard rule: include "I" and "me" last whenever you have a series of two or more subjects/objects listed together.