quadrennial 1. 四年的 2. (每)四年(发生)一次的 3. 每四年一次的事(如四周年庆祝活动等) 四年举行一次 It has been held once every four years.足球世界杯四年举办一次, 是全世界球迷的共同节日。 A common festival for all football fans all over the world, the Football World Cup, is held once every four years.
1.Olympic games is held every four years .2.Usually, trees are planted in spring .3.I think two bridges need to be built.I think two bridges need be built.(这句是把need看做是情态动词,但是好象还是没有上一句好)