how to create content by other users. Let say I am admin, i want to create stroy content as register user1?

If you're an administrator you can just set the author right in the node create/edit form in the 'authoring information' fieldset.

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"Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime." - Lao Tzu
"God helps those who help themselves." - Ben Franklin
"Search is your best friend." - Worldfallz}