Which is correct: “We attend a (five-days, five-day) seminar”?

[Joe Carillo]

Question posted in my Personal Messages box by Baklis, Forum member (May 28, 2015):

Hi Sir, which one is correct: “We attend a five-days seminar” or “We attend a five-day seminar”? Thank you.

My reply to Baklis:

The grammar rule in such hyphenated constructions is to always render the noun expressing the quantity in the singular form, so the correct construction for the usage you presented is “five-day seminar,” as in the second sentence you presented: “We attend a five-day seminar.”

Think of it this way: As a rule, a noun that by itself (particularly not in the possessive form with apostrophe-s, as in “women’s room”) premodifies another noun functions as an adjective, and adjectives typically don’t take the plural form. We can see that this applies in such examples as “car key” (we don’t say “cars key”), “plane ride” (not “planes ride”), and “table manner” (not “tables manner”). When we compound a noun to become a hyphenated quantity modifier like “five-day,” it similarly functions as an adjective; as such, it typically takes the singular form like all adjectives do. We don’t say “five-days leave” but “five-day leave,” not “200-pounds boxer” but “200-pound boxer,” not “40-storeys condominium” but “40-storey condominium,” etc.

[Baklis]

Sir, are we going to apply that rule when we use ''two-year'' or ''four-year'' as in He's studying a four-years course?

[Joe Carillo]

Yes, that rule is pretty well standard no matter what the time measure is. Whether day, week, month, year, decade, century, or millennium, the term should be in the singular form when compounded with a number by a hyphen to form a quantity premodifier, as in the following examples:

“a four-year course,” not “a four-years course”
“a five-century tradition,” not “a five-centuries tradition”
“a 20-decade conflict,” not “a 20-decades conflict”
“a 100-millennium celestial phenomenon,” not “a 100-millennia celestial phenomenon” or “a 100-millenniums celestial phenomenon”

The same rule applies when the count is for a number of people or things, as in the following examples:

“a five-man team,” not  “a five-men team”
“a 30-woman contingent,” not “a 30-women contingent”
“a 25-member choir,” not “a 25-members choir”
“a 5,000-tree mango plantation,” not “a 5,000-trees mango plantation”
“a 45-mammal menagerie,” not “a 45-mammals menagerie”

I'm sure you've gotten the hang of it by now.