use send or sent (hard) ?

首先,點解個ms話你錯呢?過去式除左可以指以前發生嘅事,重可以代表禮貌,例如公函,或者假設句.而命令係用現在式:
例:close the door please(就用現在式)
If I were a boy>>>(呢個係假設句)
你呢句嘢嘅中文係:有任何問題,都可以問我,完全係現在式.如果同朋友講,可以話
If you have qusetions, you can send me a letter.(禮貌d可以用could)

禮貌版: If you had any further qusetion, please send me a letter.

因為前半句要禮貌d,所有要用過去式,但係後面係要求,所以要現在式,如果你驚錯嘅話,認住個please,有please就即係請你而家做d乜,又已經有禮貌嘅please字,又係而家做,所以用現在式就得
禮貌嘅公文係唔用簡寫

If you have any questions,please send me a letter (e-mail is fine).

If you have any questions,please e-mail them to: ...

which of the following are wrong?
which of the following is the best?

If you have qusetions, you can send me a letter.
If you have qusetions, you could send me a letter.
If you had any further qusetion, please send me a letter.
If you had any further qusetion, please sent me a letter.

IF you had any further question, please send me a letter. Here is to tell somebody to do somthing so send is correct.
However, we should avoid misunderstand by writing the sentence . If you have............
........................

Send is correct because you said "please send me a letter", this is something you are telling someone to do, you should always use simple present tense for this.
E.g. please do this for me (insead of "please did this for me").

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send is correct=]

你的老師是對的。
這不是 tense 的問題，而是 mood 的問題。
你可以這樣看：句子的第一段，作者認為已把情況羅列清楚，那不過是假設；但如還有疑問，則應以平舖直述看：那就是用 send 的理由了。