v.(动词)这个问题已经存在几周了。误The problem has been existed for weeks.
正The problem has existed for weeks.
析exist是不及物动词,不可用于被动结构。
那样的事是从来没有过的。误Nothing like that has ever existed in the world.
正Nothing like that has ever existed.
析exist本身就有“存在于世上”的意思,因此,其后不用再接in the world。