Using a plural in English can be tough. There are many rules, and sometimes they are hard to understand and remember. A good example of a strange case is when you use a compound adjective.
A compound adjective is an adjective made up of more than one word, linked by hyphens: ex. “an English-speaking country” or “a time-saving gadget”. Some of these compound words use numbers, like “a three-year-old boy” or “a twenty-minute walk”.
I often hear students say “a three-years-old boy” or “a twenty-minutes walk”. But that is a mistake. When I tell my students, they are often confused. They ask me, “Why should I not use an ‘S’? It’s twenty minutes! It's six years!”
The reason is that “three-year-old” and “twenty-minute” are adjectives, and in English only nouns can be plural. You would never say “reds cars” or “youngs people”, would you?
Here are some more examples:
- a two-metre shark;
- a three-story house;
- a six-page document; and
- a ten-eyed monster.
