About英文文法!

兩句都啱文法，不過意思有分別，
(1) When he did that, the other kids would laugh at him.
(每)當他那樣做的時候，其它的小孩就會嘲笑他。
The sentence implies that he had done it more than once, and got laughed at every time by the other kids.
(2) When he did that, the other kids laughed at him.
當他那樣做的時候，其它的小孩嘲笑他。
The sentence only described what happened this time, without any reference to what happened what he did it at other times.




2010-03-23 15:02:23 補充：
Correction to ed_young's answer:
Last Sentence: the second "what" should have been "when":
The sentence only described what happened this time, without any reference to what happened [when] he did it at other times.

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When he did that, the other kids would laugh at him.

由於這句開首是過去式，證明了他所做的事情已經做完的了。

would 的用法︰

‧用以表示一設想事情的結果(與以上句子所用的不恰)

‧與 have+過去分詞連用，表示假想的過去的動作或事情(沒有用 have，與上述句子不同)

‧用以表示假想的現在的動作或事情(已經過去，並不是假想的)

‧用以出客氣的請求(以上句子不是請求)

‧與 imagine, say, think 等詞連用，表達試探性的意見(沒有與那些詞連用到，亦不是意見)

‧用以提出建議或邀請(不是請求等的句子)

‧用以評論持有的行為(以上句子沒有評論的語氣)

‧用以 so that, in order that 之後，表示目的(沒有用在那些詞之後)

根據以上條件分析，在此處，would 其實是可以省去，文法上也沒有錯誤的。

would ← will 的過去式(這亦是用 would 的原因)。

您可以加 would 或者可以不加也是可以的，只不過您的句子意思有點兒改了…

頭的一句是指那些小孩在他做那樣事情時，將會笑他。

另一句是他做了那樣事情，其它的小孩都笑他。

希望可以幫到您 ~~~~~

don't add 'would' will be better, because it is in past tense (happened).
but if you want to add 'will' the sentence will be like this:
If he does that, the other kids will laugh at him.