what does " Your paper seems to be sound" mean? Is it mean my essay is writing on a right track ?
If my essay appears to be good, how come the professor said , " You have many instances where there are problems."

Question

what does " Your paper seems to be sound" mean? Is it mean my essay is writing on a right track ? 
If my essay appears to be good, how come the professor said , " You have many instances where there are problems."

Answer

She accepts your essay and thinks it is valid and consistent, but doesn't mean it doesn't contain some mistakes here and there.The overall judgment is positive,however you have to consider correcting your mistakes.

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Your essay may be on the right track, but there are problems in some places. The professor should indicate where you made mistakes. If he didn't,  make an appointment to see him about your essay.

Answer

a sound argument = an argument without any logical mistakes
Your arguments and reasoning in the essay are sound, but you may have some grammatical problems.

Answer

Yes! It means that your essay appears to be good. :)