There are many people we hear using fewer and less almost interchangeably. Before we get into the words themselves, there's a very interesting concept that you need to wrap your head around before you can even begin to work out which word it is that you should be using.
Fewer can only be used when a noun can be counted. If you can say that there are two of them then you are fine to use fewer. As a general rule of thumb, liquids tend not to be counted in integers, making them, more often than not, uncountable. You can have fewer apples but not less apples.
Since fewer is used for countable nouns, then less must be used for the uncountable. Can you have three waters? Technically yes, but only if you're ordering in a restaurant. Usually water is uncountable, so you can have less water. Likewise, you can have less sand but fewer grains of sand.
Less is also used when referring to abstract concepts. You can be less successful, less efficient and less affluent but never fewer successful, fewer efficient and fewer affluent. It sounds horrible just saying it. Odds are, if it doesn't sound wrong when you say fewer, then you are probably correct in using it.
Do you have any common grammatical mistakes you feel we should address? Tell us about them in the comments below.