Usually adjectives modify nouns, but sometimes we use nouns to describe other nouns. We can use a noun as an adjective, and like all adjectives this “noun adjective” comes before the noun it’s modifying, and it always takes a singular form.
- A flower garden (a garden of flowers)
- A window seat (a seat near the window)
- A bike shop (a shop that sells bikes)
- A ten-minute walk (a walk that takes ten minutes)
- A house key (a key for your house)
- A three-course meal (a meal that has three courses)
Because the “noun adjective” is acting as an adjective, it never takes a plural form.
- Flower gardens (NOT flowers gardens)
- Window seats (NOT windows seats)
- Bike shops (NOT bikes shops)
- A ten-minute walk (NOT a ten-minutes walk)
- House keys (NOT houses keys)
- Three course meals (NOT three courses meals)