How do I know if an adjective should be hyphenated?

Here are some tips:

1. Before a noun: usually hyphenated
When a compound adjective comes before a noun and works together to describe it, it's often hyphenated:
well-known author
full-length movie
high-quality materials
old-fashioned ideas

2. After a noun: Usually not hyphenated
If the compound comes after the noun, the hyphen is usually dropped:
The author is well known.
The movie was full length.
The materials are high quality.

3. Familiar compounds can become one word over time.
Some compound adjectives start hyphenated, but if they become common enough, they may fuse into one word:
nationwide (used to be hyphenated: "nation-wide")
online (used to be hyphenated: "on-line")
There's often no hard rule here, just common usage. It's always best to have a good dictionary on hand.

4. Adverb + adjective = no hyphen
When the first word ends in -ly, you usually do not use a hyphen:
a highly rated show
a poorly written article
a completely honest answer
Because the adverb ending in -ly already clearly modifies the adjective, no hyphen is needed.

5. Noun + adjective = hyphenated (often)
When a noun modifies an adjective, it's usually hyphenated:
snow-white fur
oil-rich country

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