There is an easy rule to remember for when to use the word less and when to use the word fewer and that is that less applies to things that can't be counted whereas fewer applies to things that can be quantified. Here are examples:
Less—time, stress, pollution. San Francisco has less pollution than Beijing. I have less time now to write writing tips than I used to.
Fewer—students, cars, obstacles. Argenis was happy to have fewer obstacles in his new job than the last job. Fewer students are studying Spanish this term than in the summer.