Hyphen

Usually no hyphen is needed between a noun–noun or adverb–adjective combination:

• Football player
• Very nice girl

But to avoid possible misunderstanding when groups of compound modifiers appears before a term, we need hyphen:

• a man-eating shark is a shark that eats humans
• a man eating shark is a man who is eating shark meat

• a blue green sea is a contradiction
• a blue-green sea is a sea whose colour is somewhere between blue and green

• American-football player
• American football player

When the same combination of words follows the term it applies to, hyphens may or may not be required, depending on whether the compound constitutes an adjective or not. For example:

• American-football player / a player of American football

Hyphen is often used with weights and measures:

• A meeting of 2 days > a 2 days meeting > a 2-day meeting
• 28-year-old woman

• three-hundred-year-old trees are trees that are 300 years old.
• three hundred-year-old trees are three trees that are 100 years old.