The simple rule about when to use fewer and when to use less is that fewer is used with countable things and less is used with uncountable things. The problem is that this rule is not always true. Some English teachers will say that exceptions are the result of modern people being lazy with English. One good way to check this idea is to look at a very respectable old book, such as the Anglo-Saxon Chronicle.
According to the Anglo-Saxon Chronicle in A.D. 642, "The same year in which Oswald was slain, Oswy his brother succeeded to the government of the Northumbrians, and reigned two less than thirty years."
Years are definitely countable. There is even a number there to make that clear. The problem is that the rule is wrong. It was invented by English teachers who only got their description of English half right.
When using fewer and less there are three standard forms, not two. The first two are familiar. The third is often not taught. When teaching native English speaking children, a teacher might say "use the word that sounds right". That is lazy teaching and certainly does not work when teaching second language learners in Hong Kong.
The first form:
Use fewer if you're referring to things that you can count, such as bicycles, people or books.
Fewer parents are reading bedtime stories to their children.
At this school fewer students got all A grades on their report card.
Chinese people are buying fewer bicycles.
We sold fewer books this year.
The second form:
Use less when you’re referring to something that can’t be counted, such as homework, money, water, time, love. Things that can’t be counted don’t have a plural form.
Mr. Hung is very strict, but he gives us less homework.
I’m not that thirsty, give me less water.
I would be happier as an electrician, but I would make less money.
In Hong Kong people spend less time in traffic jams than people in Bangkok.
I love him less and less as time goes by.
The third form:
This is the form that most English teachers get confused by, both native speakers and second language learners. Less is also used with numbers of things, including measurements, such as time or distance. (Look for an actual number!)
The hiking trail is less than thirty kilometres.
He was at university for less than four months before failing out.
She can’t see that movie, because she is less than 18 years old.
Express Checkout: 10 items or less
As usual in English, there are some exceptions: Germs are uncountable (except by microbiologists), but normally come with an s on the end (plural form). However "fewer germs" is used more often than "less germs". Pants (and scissors...) normally use less, even though they are countable, because they come with an s on the end. In confusing cases native speakers often use both. That leads me to a final point. If you make the wrong choice between less or fewer there is very little chance for confusion. So if you are unsure, pick one and use it.