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If you are suing an individual, simply name the person on the form, using the most complete name that you know for that person. If the person calls himself J.R. Smith and you don't know what the J.R. stands for, simply sue him as J.R. Smith. However, if with a little effort you can find out that J stands for James, it's better to sue him as James R. Smith.
What to do if a person uses several names. Lots of people use two or more names. Thus Jason Graboskawitz may also use J.T. Grab. When in doubt, list the name the person uses most often for business purposes first, followed by the words "also known as" (or "a.k.a.") and any other names. Thus you might sue Jason Graboskawitz, a.k.a. J.T. Grab.