Think of a hyphen as mini-conjunction — that is, something that joins words together. In noun phrases like singer-songwriter and actor-director, the hyphen joins two nouns of equal weight, indicating that the person in question is both. In compound adjectives, one of the hyphenated words modifies, or describes, the other. Together, the two modify the noun.

For example, consider the phrase best kept secret. Does it need a hyphen?

(Pause for reflection.)

Yes. It’s not the best secret; it’s the one that’s kept best. So it’s the best-kept secret.

Suppose one of your classmates is celebrating a birthday with a huge party. Will it be his 25th birthday party?  Maybe, if he’s had one every year. (Without a hyphen, 25 modifies party.) But if it’s a party for his 25th, it’s his 25th-birthday party.

I’ve been known to teach a course called “Cultures of English-Speaking Nations,” about countries where English is the primary language. Without the hyphen, the phrase makes very little sense.

Now, for some examples from recent student work:

Does the following sentence need a hyphen? The widening of the U.S. trade deficit may affect the first quarter gross domestic product by reducing it.

Yes. First-quarter is a compound adjective modifying GDP — a phrase consisting of  a noun and two adjectives that has become an entity in itself. (By the way, a tighter edit: The widening of the U.S. trade deficit may affect reduce the first-quarter gross domestic product by reducing it.)

How about  this one: A 1.2 percent decline in exports and a 1.8 percent rise in imports were registered by the Commerce Department.

No. Percentages used as modifiers do not take hyphens. (Active voice would be stronger: The Commerce Department registered a 1.2 percent decline in exports and a 1.8 percent rise in imports.)