We use "less" to describe quantities that cannot be counted individually (less water, less sugar, less flour, less rice, less food, less fuel, less paper, less homework, less hair, etc.):

"less" 用来描述那些无法计算的事物，如水、盐、花、大米、食物、柴火、纸张、作业、头发等等。

This bottle contains less water than that one. (correct)

This bottle contains fewer water than that one. (incorrect!)

"Less" is also used with words such as time, money, patience, work, stress, effort, honesty, fun, love and other usually abstract nouns:

"Less" 也用来形容如时间、金钱、耐心、工作、压力、努力、诚实、忠诚、有趣、爱情等其他常用的抽象名词

I have less time to spend with my friends recently. (correct)

I have fewer time to spend with my friends recently. (incorrect!)

Tom has less money now and therefore he will drive a little less. (correct)

Tom has fewer money now and therefore he will drive a little less. (incorrect)

I have less patience than my sister. (correct)

I have fewer patience than my sister. (incorrect)

"Less" is used before a plural noun that denotes a measure of distance, time, amount, etc. :

"Less" 使用在复数名词前用来衡量距离、时间、总数等。

The town where I live is less than twenty miles from the capital. (correct)

The town where I live is fewer than twenty miles from the capital. (incorrect)

She has been working there for less than three years. (correct)

She has been working there for fewer than three years. (incorrect!)

(we use "less" because the sentence refers to a single period of time, not individual years)

Your English essay should be five hundred words or less. (correct)

Your English essay should be five hundred words or fewer. (incorrect)

I have less than five dollars in my pocket. (correct)

I have fewer than five dollars in my pocket. (incorrect)

Note: When we talk about specific dollar bills or coins we might say:

I have fewer than twenty silver dollars in my collection. (correct)

"Fewer" refers to separate (countable) items (fewer apples, fewer potatoes, fewer cubes of sugar, fewer students, fewer people, fewer drinks, fewer attempts, fewer errors, fewer rules, etc.):

"Fewer"用来形容可以分离的事物，如苹果、马铃薯、糖块、学生、人们、饮料、尝试、错误、规则等等。

There were fewer apples on the table in the afternoon than in the morning. (correct)

There were less apples on the table in the afternoon than in the morning. (incorrect)

I hope that the next time I will find fewer grammatical errors in your writing. (correct!)

I hope that the next time I will find less grammatical errors in your writing. (incorrect!)

There were fewer people than expected at the party but everyone there had a good time. (correct)

Fewer and fewer people nowadays choose to take marriage vows, most preferring to live together as partners. (correct)