Take another look at the original sentence:

Mrs. Britton was grading papers at her desk, her students were slaving over their in-class compositions, sighing heavily and flipping through their dictionaries for the correct spellings of words.

You wanted to fix it this way:

Mrs. Britton was grading papers at her desk, moreover, her students were slaving over their in-class compositions, sighing heavily and flipping through their dictionaries for the correct spellings of words.

A comma splice—a major sentence error—occurs when you have two main clauses connected with just a comma. Mrs. Britton was grading papers at her desk is one main clause. Her students were slaving over their in-class compositions is a second main clause. The conjunctive adverb moreover doesn't change the fact that you still have two main clauses connected with a comma alone.

You might want to consult the rules for fixing comma splices and fused sentences.

Go back to the sentence to try again.

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