Hyphenated compounds usually occur when the writer wants to be clear that the first adjective is modifying the second, not that both adjectives are modifying the noun. If the sentence is written with the adjectives as predicate adjectives, they are often not hyphenated.
Use a hyphen:
Generally, use a hyphen when the two adjectives are working together as a single idea.
Gina's half-finished essay sat on the counter.
My two-year-old sister loves puppies.
Some combinations generally use hyphens. They include the following
Hyphens for clarity
American English uses fewer hyphenated compound adjectives than British English. It is important to use the hyphens when the meaning might be unclear.
We bought three pound weights. (We bought three weights that each weighed one pound.)
We bought three-pound weights. (We bought several weights that each weighed three pounds.)
I saw a man eating snake. (I saw a man who was eating snake meat.)
I saw a man-eating snake. (I saw a snake who eats men.)
Hyphenated adjectives in a list
If you use two or more hyphenated adjectives in a list, you can suspend the hyphen.
I like vanilla-, strawberry-, and chocolate-flavored ice cream.
Long hyphenated adjectives
Sometimes it's fun to create long hyphenated adjectives.
The about-to-be-senior students were looking forward to their next school year.
My always-annoying-but-sometimes-cute little sister was bugging me again.
Do not use a hyphen
Adverbs combined with adjectives, especially adverbs ending in ly, do not get hyphenated.
The slowly moving train blocked the traffic. (Slowly is an adverb)
The slow-moving train blocked the traffic. (Slow is an adjective)
If the compound follows the noun, you generally do not need a hyphen. This is because when it follows the noun, the last adjective remains an adjective, but the first one becomes an adverb.
Gina's essay was half finished. (Finished is an adjective describing essay. Half is now an adverb describing finished.)
My sister is two years old. (Old is still an adjective describing sister, but two describes years and years describes old.)