I told him to stop doing that so that he wouldn't keep harassing her.
I told him to stop doing that, so that he wouldn't keep harassing her.
Are both valid? What are the differences?
That comma is not necessary, and therefore you should leave it out in ordinary writing. On the other hand, it does no real harm, and the occasional unauthorized comma livens things up, I think. It would be perfectly acceptable in dialogue to indicate a pause in speech, and that pause would make "so that he wouldn't keep harrassing her" a sort of afterthought explanation.
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That comma is not necessary, and therefore you should leave it out in ordinary writing. On the other hand, it does no real harm, and the occasional unauthorized comma livens things up, I think.
It would be perfectly acceptable in dialogue to indicate a pause in speech, and that pause would make "so that he wouldn't keep harrassing her" a sort of afterthought explanation.