One year / three years (duration) / One-year / three-year (compound modifier)
Different formsUse the plural noun without a hyphen for a duration, but a singular year in a hyphenated compound modifier before a noun.
One year / three years (duration)
Use when
The phrase states how long something lasts.
Example
“The pilot will run for three years.”
One-year / three-year (compound modifier)
Use when
The phrase comes before a noun and jointly describes it.
Example
“The agency approved a three-year pilot.”
Quick check
a three years plan, a three year plan, the programme lasted three-year
Usage note
The same pattern applies to other measurements: a six-month review, but the review lasted six months.