What if an author only wants help with a specific issue? Do I correct other problems?

While it’s important to follow the user's instructions, if the author is making an obvious and non-debatable grammatical error, definitely correct it. The author can always “reject” your changes if they don't want commas in the right place!

However, if a user asks you to correct only grammar and not make stylistic edits like upgrades/changes to word choice, voice, or sentence structure, then please follow the author’s instructions (even if it's marked as a Stylistic edit). This author may have a specific reason for writing a document in a certain way, so it’s important to respect his or her voice, even if it’s not your personal preference. 

When in doubt, follow the author's instructions. Use the comments to let them know you may have advised additional changes to something but didn’t make the change yourself because you wanted to respect his or her instructions.