These seem like simple words but can give a migraine when you try to understand their use. People usually confuse one with another, and even native English speakers can make mistakes while using them.  It would help decide whether you intend to use them for a "countable" or an "uncountable" noun.  Fewer is used for countable nouns in a comparative form.  I have fewer apples today than I had yesterday. You are making fewer grammar mistakes now than you did two months back. Whereas less is used for uncountable nouns. There is less water in my water bottle.  There is less space in the room due to the new cupboard.