有關英文書信的問題

你 tense 其實沒有什麼問題，用 present perfect tense 沒問題，但用 past tense 也可以，兩者的分別在於 present perfect tense 看起來會有強調作用，past tense 沒有，比較：


Two people have been killed.
Two people were killed.


不過在你這裡，我偏向用 past tense，感覺上較自然。


用 present tense 沒錯，因為你現在在回覆她。


至於你這封信，如果是 formal letter，語氣是合適的，但若是 informal，就好像太過正式了。

Error and corrections:-
Dear Mary,
I am sorry to tell you that my watch which I bought in April last had been lost. I believe I can't give the watch to you for Watch Exhibition on the coming Saturday.
Yours,
Steven

Dear Mary,
I am sorry to tell you that I have lost my watch, so that I cannot borrow it to you
on the coming Saturday.
Yours,
Steven

文法上大概係咁,但內容上其實可以提供埋解決方法俾MARY, 例如 I know XX has
the same watch as me, I can ask for you.

我覺得你寫得有啲怪
用詞5係太啱
如果我係你
我會咁寫:
Dear Mary,
I am sorry to tell you that I have lost my watch. I don't think I'll be able to give it to you (on) the coming Saturday.
Yours,
Steven