Depending on which grammar system you subscribe to, you may hear some very different ideas concerning when and how to hyphenate. Different style manuals conform to different rules. With hyphens, it's better to look up any case where you might be confused.
While it's hard to create hard and fast rules for hyphens, here are a few instances where you will normally use them.
When you use two or more words together as a single thought describing or modifying a noun and you put them before the noun, you should hyphenate them.
When compound modifiers come after the noun, you don’t need to hyphenate.
Hyphens with Ages
If the ages are being used as adjectives or nouns, you should hyphenate them.
- The five-year-old boy is ready for school.
But if the age comes after a noun and a verb, you don’t hyphenate it.
- The boy is five years old.
You also use hyphens when:
Writing out numbers 21 through 99, like twenty-one and ninety-nine (and everything in between).
Prefixes that come before a proper noun, like anti-American.
Avoiding confusing or awkward combinations, like shell-like or de-ice.
For more information, check out the full form of our article "When Do I Need to Hyphenate?" here.