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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.
Hunched over the keyboard for hours is a past participle phrase. A past participle phrase will often begin with an ed word like hunched. Charlie used two fingers ... is a main clause. An introductory participle phrase requires a comma when you connect it to a main clause that follows.