Take another look at the sentence:

Hunched over the keyboard for hours, Charlie used two fingers to hunt and peck out the 2,000-word research paper when lightning caused his computer to crash, Charlie wept more tears than the thunderclouds spilled rain.

When lightning caused his computer to crash is a subordinate clause, so it cannot stand alone as a complete sentence. Charlie wept ... begins a main clause. It is both legal and necessary to connect these two items with a comma.

Go back to the sentence to try again.

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