Hi Irene Both of these sentences are correct. But in my opinion, there are some subtle differences between the feelings they express. “那场 意外 发生 在 上周日 下午” is more like an answer to “When did the accident happen?”. To understand why we can put time at the end of this sentence, we need to know the purpose of this sentence. Apparently this sentence wants to emphasize the time at which the accident happened. Given that without the time at the end of sentence, the sentence becomes “那场 意外 发生”, and the new sentence only indicates “the accident happened.”, which distorts the original meaning. “上周日 下午 发生了那场 意外” is also a correct expression in Chinese. However, I lean to regard it as an oral expression, because it’s quite strange in grammar. In one way, we can regard 上周日下午 as a subject (which is kind of weird but does work in Chinese); In another way, we can take it as a modification of accident (which goes like “the accident that happened at last Sunday afternoon”). In the case of latter one, this sentence gives me a feeling that the meaning it expresses has not completed yet and there is something following this sentence. Maybe the remainder is “上周日 下午 发生了那场 意外，当时 死了 很多人”. About the rule you referred, “Time always goes before the verb”, I think it’s not an absolute rule. Precisely speaking, it’s not “time goes before verb” but “time follows subject”. The two most common places of time are at the beginning of sentence or behind the subject. Basically I think both of these usages are similar and they are interchangeable at most time. Let’s check them through some examples.

1)at the beginning of sentence a.上周五我去了一趟西班牙，我觉得西班牙女生很漂亮。 I have gone to Spain last Friday, and I think Spanish girls are beautiful. b.下周三下午我有个会要开，到时可能没时间陪你逛街。 I have a meeting next Wednesday, and in this case I’m afraid that I cannot go shopping with you at that time. 2)behind the subject a.我下周日约了我女朋友一起过圣诞节。 I’m going to date with my girlfriend next Sunday. b.我明年秋季要去美国上学。 I will go to school in United State in fall next year. Besides the usages above, sometimes we also put time at the end of sentence, but it rarely happens. Such as “我约了医生下周五”, “他去北京了上周六” etc. These are very oral expressions. This usage emphasizes the thing that happens (so we finish the majority first), and time becomes supplement to the thing we express (so after the majority finishes we add the time). I don’t recommend you use such expression because it’s too casual and illiterate. Actually the usages of time in Chinese are quite ambiguous, and I’m not sure whether I clarify them clearly. After all, hope this helps. : )

The rule taught to beginners is that the "time phrase" always comes before the verb it applies to. This is a gereralisation. What actually happens is that the "time phrase" comes before most verbs, with "most" being the crucial word here.. 發生 is an exception. There are a few, you just have to memorise them. 畫 is another. You 畫在紙上 for example.

对于我来说，汉语根本就不存在这样一条规则，放在动词前面还是放在后面都是可心的，那取决于你想表达什么！与“通常”、“大多数情况”之类的没有关系。不要用这些条条框框束缚了自己。比如： 在纸上画，和画在纸上，表面看意思差别不大，但是，它们是有细微差别的，母语者会根据不同的情况来使用它们，与规则一般放在前面，“画”是例外，放在后面这样的规则无关。 我在哪里画？你在纸上画。 我画在吗？你画在纸上。 这就是汉语，灵活、巧妙。没有那么多的死规则。

This way '那场 意外 发生 在 上周日 下午 。' or this way '上周日 下午 发生了那场 意外。' are both good, according to my Chinese friends. They do not think either one is better than the other. That accident happened last week on Sunday afternoon. Last week on Sunday afternoon that accident happened.