Referencing Guide Punctuation Remember that punctuation affects the meaning of what you write and reflects the intonation patterns which make spoken language clear.
Punctuation needs great care.
Here 'who worked hard' defines which students did well. The students, who worked hard, did well. Here all the students did well. The use of a pair of commas makes 'who worked hard' an additional comment, not a defining one. Note that in such a sentence a simple comma is unacceptable.