- A hyphen (-) is a punctuation mark that’s used to join words or parts of words. It’s not interchangeable with other types of dashes.
- Use a hyphen in a compound modifier when the modifier comes before the word it’s modifying.
- If you’re not sure whether a compound word has a hyphen or not, check your preferred dictionary.
Compound Modifiers: Two-Word Adjectives Before Nouns
A compound modifier is made up of two words that work together to function like one adjective. Without the hyphen, it’s easy to misread the sentence.
In this sentence, it sounds like you shouldn’t take down any load that is holding up a wall. A hyphen should be inserted between load and bearing to make it clear that we’re talking about walls that are bearing a load.
Generally, you need the hyphen only if the two words are functioning together as an adjective before the noun they’re describing. If the noun comes first, leave the hyphen out.
You also don’t need a hyphen when your modifier is made up of an adverb and an adjective.
incorrect]Do you expect me to believe this clearly-impossible story?[/incorrect]
Hyphens With Participles
Compound modifiers that include present or past participles follow the same rules as any other compound modifier.
Hyphen With a Noun, Adjective Or Adverb and a Present Participle
When we combine a noun or adjective and a present participle (a word ending in ‑ing) to form a unit of meaning that describes another word, use a hyphen to make that unit of meaning clear.
Without the hyphen between beautiful and looking, your reader may stumble over the sentence. Perhaps there’s a new type of daisy called the “looking flower”?
Don’t use a hyphen when the modifier comes after the noun it’s describing.
Don’t use a hyphen when you are combining an adverb and a participle.
Hyphen in a Compound Expression With a Noun and Past Participle
Compound modifiers that contain a past participle also follow the same rules as any other compound modifier. Use a hyphen when the compound goes before the verb it modifies:
Don’t use a hyphen when the compound comes after the noun it describes.
Hyphenated Compound Words
Hyphenated compound words are the ones (obviously) with a hyphen between the words. Over time, many hyphenated compounds become closed compounds—teen-ager became teenager for instance. Check a dictionary if you’re not sure whether to use a hyphen or not. Here are a few examples of common hyphenated compound words:
Closed Compound Words
Hyphenated words tend to become closed compounds (single words with no hyphens) over time. Email instead of e-mail, for example, is increasingly common. If you aren’t sure whether a words is a closed compound or a hyphenated one, check your preferred dictionary.
Open Compound Words
Open compounds are typically made up of two nouns that are used together to represent a single idea. “Open” means that there is a space between the two words and no hyphen. A good dictionary is the best place to check whether a compound is open or not.
Hyphens and Numbers
Numbers between twenty-one and ninety-nine should be hyphenated when they’re spelled out.
Hyphen in Compound Adjective With Numbers
When numbers are used as the first part of a compound adjective, use a hyphen to connect them to the noun that follows them. This way, the reader knows that both words function like a unit to modify another noun. This applies whether the number is written in words or in digits.
However, a hyphen is not required if the number is the second word in the compound adjective.
Hyphen In Compound Adjective With Fractions
When using a fraction (e.g. half or quarter) as part of a compound adjective, it should be hyphenated so the reader understands which fraction is modifying which noun.
Hyphen With Prefixes: Ex-, Self-, All-
Use a hyphen with the prefix ex- (meaning former).
Use a hyphen with the reflexive prefix self-.
Make sure you don’t confuse the prefix self- with the noun self.
When using all as a prefix, add a hyphen.
Hyphens with High or Low
When using high or low as part of a compound adjective, use a hyphen when the compound comes before the noun it’s modifying. Some examples of compound adjectives using high and low include high-level/low-level and high-impact/low-impact
from Grammarly Blog https://www.grammarly.com/blog/hyphen/