Than 和 as 應該什麼用?

as, then原本是連接詞，但它們也很奇怪，有時也能具關係代名詞用，有文法書稱它們為準關係代名詞，故它能引導出沒有主詞或賓語代詞的從句

如：Read such books as will make you better and wiser. (不可說...as it will make you better and wiser, 因為as已有it的意思）

Peter's going to join us, as was agreed last week.(不可說...as it was agreed last week)

He worries more than is good for him.（不可說...than it/what is good for him)

Don't lose your octopus card, as I did last year.(不可說...as I did it last year, 因為as已有可代替賓語it的功能)

I will buy the same watch as you have.(不可說...as you have it)

The sent more than I had ordered(不可說...more than I had ordered it)

因為as，then是這樣等別，故你的句子，１，２句as, then有了主詞it的功能
而３，４句的as, then卻有了賓語it的作用

希望幫到你！

請留意
1,2句前後部份其實在說同一個人/東西所發生的，即是說句子中主角沒有改變，不是不能
The train might be late, as it happend yesterday.
He worries more than he is necessary.
而是一種省略寫法。以上兩句文法上完全正確，但未必是英語人的講法，你想講出/寫出地道像英語人的句子便要學識那些地方可以省略或句子結構在文法的規則可以改變的地方。

3,4句前後的事是一樣的，但是表達的主角前後是兩個不同的人，文法上是有另一個subject(主體)的分句/子句，所以必須要寫出第二個主是甚麼。
3a是對you你說出情況警告/勸告
3b是i我發生的情況
4a是they他們
4b是i我

我不懂那些文法上的術語, 希望你更易明白:


問題一: 我唔明白既係,為什麼第一句不可以改為"The train might be late, as it happened yesterday." 但係第三句就可以用"as i did last year."只不是"as did last year."

happen 是發生, 是事情(晚點)的發生, 不是火汽車的發生, 故不能說 the train (it) happened yesterday. 硬要加主詞的話, 可以加 what, 變成 as what happened yesterday. 第三句情況不同, 是'我做了同樣的事', 故 I did 是正確的, 亦可作 what I did

問題二: 然後第四句也是一樣,為什麼第2句不可以改做"than he is necessary".,但第四句就可以用"than i had ordered",即是為什麼第四句可以在than之後加上"I"?

英文的 necessary 指在一些情況下必須做或需要, 應用 it is necessary, 不能說 he is necessary. 硬要用中式表達法, 那便是 He worries more than he needs to. 第四句 order 是 I 的動詞, 故可用 I had ordered. 亦可用 passive voice 變成 than what had been ordered