In his classic short book on punctuation, Mind the Stop, G.V. Carey says of the comma: ‘The writer who handles this puny little stop correctly and sensibly can probably punctuate as well as need be.’ My work as a copy-editor generally bears this out, but such proficiency is unusual. It’s a tricky mark to master.
One of the first things we learn implicitly about commas is that they’re not normally used between a subject and predicate: Jane cycles, not *Jane, cycles. They may, of course, be needed in pair form if the subject is followed by an appositive phrase (Jane, a city girl, cycles) or a non-restrictive clause (Jane, who is a city girl, cycles).
Jane, cycles is perhaps a misleading example in that the subject is short and simple, and such a mistake would be unlikely from a native-English speaker with basic education. Lengthen or complicate the subject, though, and commas begin to materialise.