If you write: the students are eighteen years old, you don't need hyphens. The second compound in that clip was twenty-two. Always use hyphens in numbers from twenty-one to ninety-nine. Twenty hyphen one (twenty-one). 

Two hundred and ninety hyphen nine (two hundred and ninety-nine). 

Now the last compound adjective we had there was last-minute. The students were going to make a last-minute decision. 

And that's the adjective last plus the noun minute, joined with a hyphen. 

Some more compound adjectives. 

 Well respected. That's an adverb, well, and the past participle of a verb, respected. And together, they make an adjective, and the two parts of the adjective need a hyphen when we write them before a noun. So it's a well-respected politician, with a hyphen: well hyphen respected politician (well-respected politician). 

But in a phrase like the politicians were well respected we don't use a hyphen, because the adjective comes after the noun, not before. 

And that rule is the same for three-word compound adjectives like out-of-date. 

So, the phrase out-of-date attitudes has hyphens because the adjective is before the noun, but the phrase their attitudes are out of date doesn't have hyphens. 

 And one last rule is that we never use hyphens in compound adjectives that have an adverb which ends in -l-y. 

So in phrases like a carefully written letter we don't use hyphens. Now let's talk about compound nouns. The phrase secondary school is a compound noun - and there's no hyphen in it. Most compound nouns are written as two separate words. And it's time for a quiz! Number one: What's the compound adjective in this sentence and does it need a hyphen? We were late because of the slow-moving traffic. Slow-moving is the compound adjective. Forty hyphen mile run (forty-mile run). The teacher was very well liked. Is there a hyphen in well liked? We don't need a hyphen there.  And before we go, here's a vocabulary tip. When you are reading, make a note of compound adjectives and nouns with - and without - hyphens. Keep a list and check it regularly. There's more a points to take away: There are a lot of ways to make compound adjectives. Most compound adjectives have hyphens, but some do not. Compound adjectives with hyphens in them include: adjective/adverb + present participle: a hard-working student, a good-looking man ages and numbers when used before a noun: eighteen-year-old students, a two-door car compound numbers from twenty-one to ninety-nine: fifty-two, seventy-eight compound adjectives formed in other ways, for example adjective/adverb + noun a last-minute decision, a one-way street, full-time staff or noun + adjective: a world-famous athlete adverb/noun + the past participle of a verb when used before a noun: well-respected politicians, a battery-operated toy three-word compound adjectives when used before a noun: an out-of-date hairstyle, an out-of-work father Compound adjectives without hyphens in them include: adverb/noun + the past participle of a verb when used after a noun: the politicians were well respected, the toy was battery operated three-word compound adjectives when used after a noun: that hairstyle is out of date, his father is out of work. compound adjectives made with an adverb ending in –ly, both before and after a noun: a carefully written letter, the letter was carefully written. Compound nouns do not usually have a hyphen: secondary school, swimming pool, ice cream But a few do.