If knowing when to hyphenate adjectives (or not) is something you’ve just never managed to figure out, don’t worry – you are far from alone! Here we break down a simple test you can use to see when you need to apply this grammar rule to hyphenate adjectives in your writing.
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The basic rule for when to hyphenate adjectives
In simple terms, your rule follows…but don’t worry if you don’t get it yet – there are examples below!
When two words work together to form one adjective that modified a noun, those two words are hyphenated, EXCEPT when the first word ends in -ly.
What does it mean for two words to work together as one adjective?
To show what we mean here by two words that work together to form one adjective, we will talk about cells.
To test whether we need a hyphen, let’s start with the phrase:
“big round cells”.
To test whether these words work together as one adjective, first divide the two words and see if the phrases still make sense:
“big cells”
“round cells”
Here, both of those phrases are correct and make sense, and none of the meaning here is lost.
Therefore, these words ARE NOT working together as one adjective and are in fact two separate adjectives.
When the sentence still makes sense when the adjectives are separated, you do not use a hyphen.
Let’s look at another example and use the phrase:
“insulin deficient cells”
Here, we can split this up into two phrases again to test whether or not these adjectives work together:
“insulin cells”
“deficient cells”
Here, hopefully you see the difference and notice that now our two separate phrases DO NOT make sense, so these adjectives are not two separate adjectives modifying the noun.
In fact, this sentence ONLY makes sense when the two words are together.
Therefore, when the sentence wouldn’t make sense if the adjectives are separated, the adjectives need to be hyphenated (unless the first ends in -ly).
If you wanted to put this altogether, we can have:
“big, round, insulin-deficient cells”
When phrases are only sometimes hyphenated
Sometimes writers get confused between when to hyphenate two words that are often found together in their paper but used in different capacities.
The example of “stem cells” is a common one.
For example, let’s discuss
“stem cell screening”.
The noun here is “screening”, and the words “stem cell” are modifying that noun to describe what type of screening.
If you perform the test we did above, you would get
“stem screening”
“cell screening”
Obviously here, the phrase does not retain its meaning when these terms are separated, so they are working together as one adjective.
Therefore, you would want to write:
“stem-cell screening”
When the phrase isn't an adjective at all
Here, this phrase is written such that the noun is “screening” and “stem cells” is acting like an adjective that describes what type of screening.
However, “stem cell” can act as a noun in its own right (or, technically, “cell” is a noun modified by the adjective “stem”, but we don’t use a hyphen with only one adjective).
For example, we can rearrange the above phrase to:
“We performed this screening on stem cells…”
Here, since “stem cell” is a noun that isn’t modifying
Confusing point: when the adjective includes a number
This point seems to also confuse writers when the adjective contains a number.
For example,
“5 mL sample”.
Here, “5 mL” works together as one adjective to describe this sample. Your meaning here would not be retained if you described this as a “5 sample” or a “mL sample”, so this must be hyphenated:
“5-mL sample”.
Let’s rearrange this sentence to highlight the above point as well:
“Each sample is 5 mL.”
Here, “5 mL” is no longer serving as one adjective and so is no longer hyphenated.
Exception: when the first word ends in -ly
Now let’s finish with one example of the -ly exception.
Let’s take the phrase:
“commercially available compounds”.
Here, separating these words will give you two phrases that lose the meaning of the original phrase, so they must work together.
BUT that doesn’t even matter here because with the -ly at the end of the first word, this is never hyphenated.
So that’s it – that’s everything you need to know about hyphenating adjectives.
Hopefully it feels like one of the universe’s greatest mysteries has been unveiled for you, and you never need to think of this again when writing.
