How to make compound adjectives with numbers (75% make this ONE MISTAKE!)

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I recently posted this question to a group of students on Facebook, and only 25% of the responses were correct.

I’m going on a ___ trip.

a) three day
b) three-day
c) three days
d) three-days

In this lesson you’ll learn why so many people got this wrong, so that you won’t make the same mistake yourself.

What are adjectives again?

This lesson is about adjectives. Adjectives are words that describe nouns or pronouns. They tell us about people, or places, things or ideas.

They live in a big house.

The adjective big describes the noun, house.

He is tall.

The adjective tall describes the pronoun he.

Compound Adjectives

Adjectives can also be made from multiple words. These are called compound adjectives.

Here are some examples:

I have a part-time job.
He is a well-known actor.
The hospital uses state-of-the-art equipment.

In these examples, there are hyphens in between the words that make up the compound adjectives. This is often the case when the adjective goes before the noun it describes. These hyphens show that these words are working together as one unit.

Compound Adjectives with Numbers

You can also make compound adjective by combining a number with a noun. At first, they might look like they are incorrect. We’ll get to that in just a moment.

Here are some examples of compound adjectives with numbers:

She has a five-dollar bill.
I live in a one-bedroom apartment.
The student wrote a 20-page report last night.

Did you notice anything strange about these adjectives? They all use the singular form of the noun. When the nouns are combined with numbers to make an adjective, we never use the plural form.

Let’s look at each one of these examples.

Example 1

She has five dollars.  

There is no compound adjective in this sentence. The number five describes the amount of dollars she has. There is more than one, so we need to use the plural form of dollar.

She has a five-dollar bill.

Here, we combine the number five with the word dollar to describe the bill. These words work together as an adjective, so we use the singular form of the word dollar. It might feel strange to use the singular form of the noun at first, but it gets easier once you become familiar with it!  

What would the sentence be if she had two of them?

She has two five-dollar bills.

We put the “s” on the word bills because there are two.

Example 2

The apartment has one bedroom.

In this sentence, the word one tells you the number of bedrooms the apartment has. There is only one, so we don’t need to use the plural form of bedroom.

I live in a one-bedroom apartment.

Here, we combine the number one and the noun bedroom to form an adjective to describe the noun apartment.

What if you want to describe two apartments for rent, and both apartments each have one bedroom?

There are two one-bedroom apartments for rent.

We put an “s” on the word apartments because there’s more than one. We don’t put an “s” on the word bedroom because it is combined with the number one to make an adjective.

Example 3

The student wrote 20 pages last night.

In this example, the number 20 describes how many pages the student wrote. We need to use the plural form of the noun, pages, because there is more than one.

The student wrote a 20-page report last night.

Here, we combine the number 20 with the word page to describe the report. These words work together as an adjective, so we use the singular form of the word report. 

 What if the student wrote two reports and  each report had five pages? This what the sentence would look like:

The student wrote two five-page reports.

Rules for using numbers in writing

One thing that I should mention is that If we are using a number that is 10 or greater, it is most common to use the numerals (e.g., 10, 50, 100). If the number is less than 10, we write it out in words (e.g., one, two, three). This is a general rule for writing.

How to form compound adjectives with numbers – review

When you want to make a compound adjective out of a number and a noun, use the singular form of the noun and connect the words with a hyphen (-). If the number is less than 10, write out the number (e.g., five-star hotel). If the number is 10 or greater, use the numeral (e.g. 12-hour shift).

Almost forgot about the question…

I’m going on a ___ trip.

a) three day
b) three-day
c) three days
d) three-days

Do you think you know the right answer now? Let me know your answers in the comments.

Conclusion

You can see why these adjectives can be so confusing for English language learners of all levels, and I hope you understand how to use them correctly when speaking and writing.

Are you ready for a challenge? Take the compound adjectives with numbers quiz. Share your results with your friends, and see who can get a higher score. If you get 10 out of 10, you’ll be a grammar superstar.

If you have any questions about anything that I covered about today? Let me know in the comments below.