What is the difference between "had done" and "had did" ?
John had done this/it
John had did this/it
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In this case, as long as we are referring to an action in the past, we can say, "John had done it.", the past perfect form.
However, "John had did it" is incorrect. To form the past perfect, we need to use the past participle of "To do", which is "Done"
Note that using "This" implies a connection to the present, so the present perfect is required.