Once and for all, when do we use ‘fewer’ or ‘less’?

This very interesting observation about the use of comparatives was posted in Jose Carillo’s English Forum recently by a member who goes by the username Jhinx22:

“People often don’t know when to use ‘fewer’ and when to use ‘less’ in a sentence. ‘Fewer’ is used when referring to people or things in plural, as in this sentence: ‘Fewer students are opting to study science-related subjects.’ ‘Less,’ on the other hand, is used when referring to something that can’t be counted: ‘People want to spend less time in traffic jams.’