Actual Work vs. Work vs. Duration

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I am relatively new to MS Project and I am working on a complicated
plan in which I would like to do the following:

1. Estimate out x numbers of hours for a bunch of tasks
2. Record the actual number of hours it took to complete the tasks
(without affecting the original estimate)
3. Mark a task as complete even if the actual hours were less then
estimated (again without affecting the original estimate)

I think that this would seem simple. I have done the following:

1. Used "duration" to track estimated hours (probably not correct but
I can't see where this would be better placed)
2. Used "actual work" to track actual hours

Problem is that when I make a tasks duration say 12 hours and it only
takes 8 (I put this value in Actual Work), and then I mark it complete
(%Complete) the value for Actual Work is converted to 12.

Any help would be greatly appreciated.

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