I am sorry that I write back to you (for) I (have received）your letter for three weeks.为什么用forI am sorry that I write back to you (for) I (have received）your letter for three weeks.为什么用for 为什么用 have received?
问题描述：
I am sorry that I write back to you (for) I (have received）your letter for three weeks.为什么用for
I am sorry that I write back to you (for) I (have received）your letter for three weeks.
为什么用for
为什么用 have received?
答
for引出的是原因状语从句。
答
非常抱歉，（for的意思因为）我（已经收到你信件）三周了才给你回信。
for表 原因的表语从句，意思是因为。
答
非常抱歉,（for的意思因为）我（已经收到你信件）三周了才给你回信.
for表 原因的表语从句,意思是因为.
至于have received是过去完成时,强调已经收到来信!