You use ski to refer to things that are concerned with skiing.
the Swiss ski resort of Klosters.
瑞士的克洛斯特斯滑雪胜地。
a private ski instructor.
一位私人滑雪教练。
skis
Skis are long, flat, narrow pieces of wood, metal, or plastic that are fastened to boots so that you can move easily on snow or water.
a pair of skis.
一副滑雪板。
skis , skiing , skied
When people ski, they move over snow or water on skis.
They surf, ski, and ride.
他们冲浪、滑水和骑马。