happen有意外發(fā)生的意思，相當(dāng)于漢語(yǔ)中的“碰巧發(fā)生”，例句：I don't know what happened.我不知道發(fā)生了什么（碰巧）。appear的意思就是“出現(xiàn)；顯現(xiàn)”，沒(méi)有什么特別的強(qiáng)調(diào)碰巧，就是正常使用就行了。

happen的相關(guān)例句

All these changes happened over a period of time.

所有這些變化都是在一段時(shí)間內(nèi)發(fā)生的。

You must on no condition tell them what happened.

你決不能告訴他們所發(fā)生的事。

This is the second time it's happened.

這已是第二次了。

You don't happen to know his name, do you?

你不會(huì)碰巧知道他的名字吧？

He gave me a guarantee that it would never happen again.

他向我保證這種事情絕不會(huì)再發(fā)生。

appear的相關(guān)例句

This problem first appeared in the inner cities.

這個(gè)問(wèn)題最初在市區(qū)中心出現(xiàn)。

Your name will appear at the front of the book.

你的名字將出現(xiàn)在書(shū)的前頁(yè)。

It would appear that this was a major problem.

看來(lái)這是個(gè)主要問(wèn)題。

The magazine will appear in a new design from next month.

從下月起這本雜志將以新的設(shè)計(jì)問(wèn)世。

The president appears likely to change course on some key issues.

總統(tǒng)看起來(lái)可能要在某些重要問(wèn)題上改變方針。