大学英语：to be 可以加在seem,prove,remain,appear等系动词和其后的名词或形容词之间，但也可以

The task proved (to be) impossible.

They seem (to be) such friendly people,but they never replied to our invitation.

What has happened remains (to be) a mystery.

The problem first appeared (to be) unsolvable,but later we came up with a very good solution.

注意：在seem,appear等词后，可加其他非 to be 的不定式，但这时，它们则不是系动词而是不及物动词。