句子错误i can't decide ( ) the best time is to get there
应为I can't decide ( ) is the best time to get there.
应填which,翻译为：我无法决定哪个是到达哪儿的最佳时间.
when无法使用的原因,看例句：
(Which) is the best time to go to Canada?
=(When) is (the) best to go to Canada?
what什么,是对什么东西来提问,但这里有对象,是time,所以不可以
whether 是否,要用这个词的话,句子就应该是完整的：I can't decide whether it is the best time to get there.