It occurred to sb that…是英语中的句型搭配，意思是“某人（突然）想到……”。

分析：it为形式主语，that从句为真正的主语；occur to sb 的意思是“（主意或想法突然）浮现于脑中; 被想起，被想到”。又如：

It occurred to him that he hadn’t eaten. 他想到他还没吃东西。

It occurred to me that he might be lying. 我忽然想起他可能是在说谎。

It occurred to me that he was trying to conceal something. 我突然想到他是在想法隐瞒什么。

It occurred to me that I could have the book sent to me. 我想到可以让人把书寄给我。

For one dreadful moment，it _______ to me that if someone had picked the receipt up he could have easily claimed the case already.

It occurred to me that if someone had picked the receipt up he could have easily claimed the case 

回答：

It occurred to me =我突然想到/想起that从句所指的事件.happen不能表达这个意思，因此不能使用happen.

一篇完形的文章，大概讲一个人去站台领行李。请问为什么不用 happen 用 occur 呢？

首先同意龙南老师的解答观点，再次我补充一点我的一些看法。

不及物动词occur ,happen一些相同和不同之处可以归纳为以下几点

相同点为以下几点

【1】happen ,occur 都是为不及物动词

【2】happen ,occur 都可表示动作意外地，偶然性地发生

【3】happen, occur，都可以以形式主语it开头并真正主语放在句末

【4】都不可以使用被动语态当中

occur不同点归纳为几点

【1】It occurred to sb that 从句，表示某事突然被某人想起；that从句所指的这个事件突然被某人想起/想到；某事突然出现在/浮现在某人脑海里

It occurred to me that I could have the book sent to me. 

我突然想到可以让人把书寄给我。

It did not occur to me to check my insurance policy.

我没有想到要核实我的保险单条款。

Didn't it occur to you that he was lying?
你当时没想到他在撒谎吗？

Has it ever occurred to you that this old book might be of great value?

你想到没有,这本旧书或许有很大的价值?

It never occurs to me that he was only acting a part.

我从来没有想到,他只是在弄虚作假。

It never occurred to me that I had seen her before somewhere.

我想不起来以前我在哪儿见过她。

Did it occur to him that lookers-on see most of the game?

他可曾想到旁观者清吗?

It occurred to me that I had left the door unlocked.

我忽然想起来忘了锁门了。

It occurred to her that she might adopt a homeless child.

她可能想收养一个无家可归的孩子。

It occurred to him that he hadn't eaten anything since the night before.

他想起他自前天晚上以来一点东西没吃。

【2】sth occur to sb 某事被某人想到/想起

The same idea had occurred to Elizabeth.

伊丽莎白也想到了这个主意。

Such a simple explanation never occurred to me!

我从未想到如此简单的解释。

A wonderful idea occurred to him.

他突然想到一个妙主意。

As soon as that thought occurred to him, he felt worse.

一想到那一点,他的心情就变糟。

It occurred to my mind.我当时想到了这件事。

His name did not occur to my memory.

我当时没有想起他的名字

【3】sth occur to sb to do sth =It occur to sb to do sth做某事被某人想到/想起；某人想起/想到做某事

Didn't it occur to you to close the window?

难道你没有想到去关窗户吗?

Didn't it occur to you to phone them about it?

关于这件事你难道就没想到给他们打个电话?

It occurred to him to tell the colonel of the problem.

他想起要告诉上校那个问题。

It never occurred to me not to endeavor to do so; I was politically minded by nature.

我从来没有想到过要打退堂鼓,我天生就是搞政治的。

【4】sth+occur 某事发生

That plane crash occurred only minutes after take-off.
空难在起飞几分钟后就发生了。

A connection error might occur.
可能会出现连接错误

The crash occurred when the crew shut down the wrong engine... 机务人员关错了引擎，随即飞机坠毁。

【5】sth+occur 某事出现，存在

These plants occur only in tropics.   
这些植物只生长在热带。

That sound doesn't occur in his language so it's difficult for him to pronounce.   
他的语言里没有那个音,因此他很难发出。

Misprints occur on every page.   
每一面都有印刷错误

happen不同点归纳为几点

【1】It happen that 从句

It happens that his elder sister is a close friend of mine.   
恰好他姐姐是我的一个好朋友。

It happened that the harvest was bad that year.   
恰好那年收成不好。

It so happened that I had no money with me.   
碰巧我身上没带钱。

It so happened that he had to be present at a meeting.   
他碰巧得去参加一个会议。

【2】It happen to sb that 从句 某人碰巧做了某事/某事发生在某人身上.

It happened to me that I had been away when he called.

他打电话时,我正好/碰巧不在家。

【3】sb/sth happen to do sth 某人/某物碰巧做某事

I happened to see her yesterday.
我昨天碰巧见到她。

I happen to have the official statistics with me.
我碰巧身边有官方的统计数字。

You happened to be out when I came to your house.   
我到你家时,你正好出去了。

He happened to be going that way too.   
他恰好也正往那边走。

She happened to have just finished reading the book.   
碰巧她刚读完那本书。

Do you happen to know her telephone number?   
你知道她的电话号码吗?

We happened to discover we had a friend in common.

我们凑巧发现我们有一个共同的朋友。

I looked in the nearest paper, which happened to be the Daily Mail.

我仔细看了一下离自己最近的那份报纸，正巧是份《每日邮报》。

【4】sth happen to sb 某人发生了某事；某事发生在某人身上

It's the best thing that ever happened to me.

这是我所遇到的最好的事情了。

The boy is crying.What has happened to him?

那孩子在哭,他怎么啦?

If anything should happen to you, follow my advice.

万一你碰上什么意外,照我的话去做。

The brakes failed.Has that ever happened to you?

车闸失灵了,你以前发生过这种情况吗?

What happened to him next?后来他怎样了?

Be careful; don't let anything happen to that child.

小心些,别让那个孩子出任何事。

A bad accident happened to that family.

那家人遇到了极其不幸的灾祸。

What has happened to your hand?

你的手怎么了?

【6】sth happen 某事发生了/发生了某事

It happened under my eyes.   
我亲眼看见这事发生的。

How did this happen?   
这事是怎么发生的?

The accident happened at three o'clock.   
事故是3点钟发生的。

A crowd soon collected at the crossroad to see what had happened.  
十字路口上很快就聚集了一群人,都想看看出了什么事。

【7】as it happens〔happened〕碰巧

We met her only yesterday, as it happens.

我们只是在昨天才与她碰巧相遇。

They were all out, as it happened.

真巧,他们全出去了.

happen这单词有这样的解释：1:（偶然地）发生

                                                2:（作为结果）发生

而occur有：1:发生（不及物动词）

                    2:被发现；出现；存在

                    3:被想起，被想到

题目中一篇完型的文章，大概讲一个人去站台领行李：句子中的领行李已经是被动的动词，大概理解下来东西是被发现；出现；（本来就）存在的东西，不是忽然下子变出来的东西所以用occur