the letter reached me the day before yesterday. 信不會主動到達我,是被動的.為什麼不可以寫 the letter was reached me the day before yesterday呢..是否可以增加一些例子幫助我了解.?

你需要考慮的是動詞reach的主詞可以是物嗎，而不是letter會不會自己動(其實在擬人寫法時，物有時也是可以自己動的）。查英英字典就可以解決這個問題：
When someone or something reaches a place, they arrive there.
When the bus reached High Holborn, Tony rang the bell and they jumped off together.
Collins Cobuild English Dictionary for Advanced Learners pp.1274~5

由此可見，reach（=抵達= get to）的主詞可以是物的。

The letter reached me the day before yesterday.(本句無法將原動詞reach改寫成被動態，因語意不通)
= I received the letter the day before yesterday.
= The letter was received by me the day before yesterday.
(He probably mailed me the letter a week ago.= The letter was probably mailed to me a week ago.)

The letter was reached me the day before yesterday.這樣寫是錯誤的。如果用被動的reach來寫，怎麼還能再有一個受詞me呢?被動態的句子裡，主詞letter就是動詞reach在邏輯上的受詞。此外，就算把me拿掉也是不通的。這跟reach這個動詞的用法有關。

動詞reach的被動用法(注意語意):
He can be reached at this number.＝打這個號碼就可以跟他連繫上。
Men are often reached by flattery. =人常為恭維的話所動。

The letter=it=pronoun used as arbitrary subject (decided by random)as a that thing,group,idea already mentioned etc.
reach= v.= to arrive at or come as far as; get to; often after much time or effort;
Further eg:-After several changes of address, the letter finally reached me.----past tense., which is the form of a verb that shows the act or state described by the verb happened or existed at some time before the present moment.
eg:-I reached the end of the short book.
eg:-The legislative council news only reached me two days ago.
eg:-She has reached the age of 25 !
eg:-The sales of iphone7 has reached record level in Korea.
eg:-The two sides failed to reach agreement in the Council.
eg:-The snail reached the vegetables on the ground.
eg:-The ship reached to the horizon two days ago.
eg:-The ladder won t reach as far as the outside window.
eg:-The ladder reached down to the road !
eg:-Could you please reach the letter from the mailbox ?!
eg:-The monkey reached out for the fruit two days ago !?
eg:-The letter reached me two days ago.= past tense.

被動語態要從受詞的角度去考慮, 而不是從主詞的角度. 所以你以"信"的角度來看, 如果要用被動, 那它的身分是受詞, 所以會有另一個"主詞"來做這個動作:
Someone sent/delivered/mailed/brought the letter to me. =>
The letter was sent/delivered/mailed/brought to me.
所以"信"可以[被(送/寄/帶)]
可是reach是"到"的意思, 即使是中文也沒有"被到"的說法, 英文也沒有. 這個動詞強調的是[從有距離變成沒有距離]的狀態變化, 所以不用被動.

1. 要是在英語裡面 每個動詞 都要區分 <主詞 可以是否 是 物>的性質 的話 老美 都要自殺200次了.
2. 這位版主 問這隻問題 也是想要 讓我來 臭罵他一頓. 當然不是 只有這位版主才是醬子 幾乎所有我教過的台灣人 都是醬子: 就是 <當 在學英語的時候 整個人 都變成 語言博士 連走路 右轉彎 都要先查閱 說明書 13次 以後才知道 要怎麼做> 你拜託一下 用一點 人類的基本常識來使用英語 好嗎?

<信不會 自己送 所以要用被動> 這種 白目理論 3歲以上的人 會說得出來嗎?
真的是醬子的話 <錢 也不會自己花> 那你為神會說 <錢 都花完了>? 反而少說 <錢 都被花玩了>?
<飯 也不會自己吃> 那位神 大家都說 <飯 早就吃完了> 反而少說 <飯 早就被吃完了>?
<屁 也不會自己放> 那位神 大家都說 <屁 放得好大聲> 反而少說 <屁 被放得好大聲>?

3. 是 中語 特別 聰明 才會醬子?還是阿多仔 龜毛 主動被動 上 斤斤計較?
當然不是 醬子 你引用的句子 才會醬子寫

4. 所有人類語言 都是醬子:

當 <動作人>(actor)是誰不重要的時候 <物> 就會 取代 <人> 而變形為<主動>
A) 例子:
<錢>都花玩了 因為<動作人>(誰花的)並不重要 所以 <錢> 就取代 <人> 而變主動

5. 這 <文法/龜澤>是應該 <博士班>的人才應該知道的.其他人會用(而且每天都在用)但是 不應該要知道的.

reach vt.及物動詞 釋義1.到達 2.(餘略) 例1. The letter reached me the day before yesterday. 例2. I hope this letter reaches you. (我希望你能收到這封信。) 例3. Your letter never reached me. 例4. The news of his death soon reached every part of the world. 例5. The village is easily reached by bus. (乘坐公共汽車很容易到達那個村莊。)