Interested in Language
By swimming quickly, the lifeguard was able to save the drowning child.
Is the comma after 'quickly' obligatory?
Yes, it is necessary.
If you say, "The lifeguard was able to save the child by swimming quickly," then no comma is necessary.
Always use a comma after an introductory phrase.
John didn't go to school because he was sick.
Because he was sick, John didn't go to school.
I am always nervous in front of an audience.
In front of an audience, I am always nervous.
Outside of a dog, a book is man's best friend.
Inside of a dog, it is too dark to read.