Passive Voice

首先, 你兩句的例句都是「被動形式」, 即 passive voice, 但係時式就應該轉用 past tense, 因為事情己過去。
Past tense
He was taught by me. 他是由我教導的。
The tree was cut two days ago. 那棵樹兩日前已經被人斬不來。
http://tw.dictionary.yahoo.com/search?ei=UTF-8&p=by
第一句例句中, by = 解「被/受到」
但係英文跟中文係兩種不同的語言, 亦不可能就每一個字噤去翻譯。
第二句雖然無by字, 但係句中tense的用法已表現到佢係passive voice, 而唔係由某一個字去解「被」。
用法係: auxiliary verb 加上 (main verb用於) past participle
即是 was taught 或者 was cut
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Active voice 再加上 Past tense 的用法:
I taught him.
Someone cut the tree two days ago. (因為不知誰人做, 所以只能用someone)
要分辨main verb是否過去分詞, 就要多查字典啦!
http://tw.dictionary.yahoo.com/search?ei=UTF-8&p=taught
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要運用Passive voice, 都有禁忌。
http://owl.english.purdue.edu/handouts/grammar/g_actpass.html
1) 在同一句句子中, 切勿同時用Active voice和 Passive voice.
例如:
Many customers in the restaurant found the coffee too bitter to drink, but it was ordered frequently.
=> Many customers in the restaurant found the coffee too bitter to drink, but they still ordered it frequently.
應該用"they" (Active voice) 做 subject, 而唔係用 it (Passive voice)。
3) 要注意正確選擇 subject
To save time, the paper was written on a computer. (Passive voice)
=> To save time, Kristin wrote the paper on a computer. (Active voice)
其實,"To save time" 和 "wrote the paper on a computer"都係同一個人(Kristin) 做的, 而唔係paper。所以切勿用錯subject。

圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_1.gif

其實"by"本身係有"被"同"用"既意思
eg.He is taught by me 他是被我教的
呢句, "他"係被動者, "我"係主動教佢既人(通常主動者都係會郁果個)

The apple was cut by a knife 個蘋果係被刀切開的
呢句, "蘋果"係被動, 被我切開, 所以係被動句

相反, active voice (主動句) 係將重點放係主動者身上
eg.I teach him. 我教他
i use a knife to cut the apple.我用刀具切蘋果

Passive Voice 基本上冇一個指定既用字
但係一句句子上出現"by"大多數都係被動句(passive voice)
當我地用passive voice個陣, 都要將個verb轉is/are/was/were+ present perfect tense

一句句子用paassive voice 寫既, 個subject 係收到個verb既行動表達
即係話個subject係被用既。

你好。

He is taught by me <--- 正確!! 呢句個 "by" 解作 "被", 全句=他是被我所教導的。
其實Passive Voice無特定邊個字先解作 "被" 既意思架, 有時無主角既。

例子如下:
1) Those dog was secured by Dr Lo = 那頭狗經盧醫生治療後康復。
2) The tree is cut two days ago. = 這棵樹已在兩天前被斬下。
3) This school was built since 1997 = 這學校建於1997年的。
4) That door was opened for one hour = 那對門已被開啟了一個小時了。

多練習、快進步, 謝謝。

You can go to www.yahoosearh.com to look.