verb+noun

除了001的答案是道地的英文說法外，你似乎堅持要用end作為verb的話，三句也不對，正確的grammar是：

I can't believe the exam has ended.

其他同類的寫法/講法有：

I can't believe you have done this.
I can't believe you have made such serious mistake.

所以記住用can't believe就用perfect tense都應該錯不了。

I think this way is better:
"I can't believe that exam is over" or "that" can be omitted ->
"I can't believe exam is over"

You ask: noun後面系唔系唔可以有verb??....in the contrary, verb must follow noun in order to make a complete sentence.