點解students唔使加 ' ?

All the error samples are real – they were produced by Hong Kong students writing in English.

The above sentence can be rewritten as:

All the error samples are real - they were produced by Hong Kong students [who were] writing in English.

or

All the error samples are real - they were produced by Hong Kong students [who wrote] in English.

Sometimes, it's okay to omit words in English to improve the flow of a sentence. It will require a far wordier explanation and several more examples to illustrate my point - may be some other time!

Also, it would be wrong to add an apostrophe after "students" because, then, you are saying:

Hong Kong students' writing in English produced the error samples.

which doesn't make sense.

2007-01-05 12:03:53 補充：
魯效陽 （[email protected])2007年1月5日帶有限定子句的複合句，常可利用分詞(現在分詞或過去分詞)，而改成簡單句，二者意思相似，例如：There are many patients who are waiting outside.(複合句)There are many patients waiting outside.(簡單句)Those who have been chosen as representatives must come.(複合句)Those chosen as representatives must come.(簡單句)

其實你既意思係想寫 "學生的文章". 理論上要加個 "'S" OR " ' " 比個NOUN
但係呢個係 ENGLISH 既 TRICKY既地方~

因為你個 " HONG KONG STUDENTS" 係形容緊個 "WRITING"呢個字~ 而 REFERRING TO 第二樣野 既 NOUN 已經有 ADJ 既用途~ 即例如最普通的 "PRETTY WOMAN" 的PRETTY 一樣~ 我地係唔會係 將 PRETTY WOMAN 寫成 PRETTY'S WOMAN~ 因為PRETTY 係形容詞, 形容緊個NOUN (WOMAN) PRETTY....

而呢個CASE, 一樣~ "HONG KONG STUDENTS" 就係形容緊個WRITING, 係有指出 個WRITING 既出處~ 所以咁寫係ARM 的!!!

因為students已經係複數,唔會寫成咁students's而係寫students'
只加'係後面,咁就已經係解學生的文章^^