Definition of tragic hero:
Achilles is the main character here, and so he's the tragic hero. Go through the definition (above) and make sure you are clear on why he's the tragic hero.
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This is an excellent webpage explaining various aspects of this epic poem as you study it.
This will help you with "theme."
This is where you can find a good translation of the epic. To me, the overall theme has been clearly stated in the first sentence! That theme is carried all the way through the story.
More information on Iliad: