For one dreadful moment，it _______ to me that if someone had picked the receipt up he could have easily claimed the case already.
一篇完形的文章，大概讲一个人去站台领行李。请问为何不需要 happen 用 occur 呢？
“It occurred to sb. that...”表“某人忽然想起或想到某事”；“It happened to sb. that...”表“某事碰巧发生或即将来临在某人身上”。比较：
It occurred to me that we should get in touch with the manager. 我忽然想起大家应该与经理联系。
It had never happened to me that I was unable to focus on the match. 我没办法专注于比赛，这种现象以前从未在我身上发生过。
1. happen“发生”，一般用词，指具体客观事物的“发生”，具备偶然性和不可预见性。如：
If anything happens to him, let me know. 假如他遭遇任何事情，公告我。（不可以用occur）
2. occur“发生”，较正式的用词。一般所指的时间和事件都比较确定，含有“按计划使某事发生”的意味。但用于具体事物时，常可与happen互换用。如：
The time when a solar eclipse will occur can be calculated. 日食发生的时间可以推算出来。（不可以用happen）
The May 4 Movement happened / occurred in 1919. 五四运动发生在1919年。
3. 表示“恰好”、“碰巧”的意义时，不可以用occur。如：
It happened that I saw him on my way home. 我碰巧在回家的路上看到他。
I happened to see her in the street yesterday. 我昨天碰巧在大街上见到她。
4. occur to sb. 表“某人忽然想起”的意思，不可以用happen代替。如：
A good idea occurred to her. 她忽然想到一个好主意。
ItoccurredtomethatIhadforgottenmykeys.我忽然想起了我忘了带钥匙。
