After you've seen the following sentences, you'll find out how great effect a little hyphen can bring about!

Twenty-one-year olds (二十一歲的人)
Twenty one-year olds (二十個一歲的人)

Dead Sea fish are her favorite. (死海的魚)
Dead sea-fish are her favorite. (死的海魚)

His car crashed into a frozen goods-truck. (雪藏了的貨車)
His car crashed into a frozen-goods truck. (運送雪藏物品的貨車)

He is ill-tempered and-educated. (脾氣又差,又沒有接受過教育)
He is ill-tempered and educated. (脾氣差勁,但接受過教育)

She brought a second hand-mirror. (第二面有手柄的鏡子)
She brought a second-hand mirror. (一面二手鏡子)