當你收到這封信時，我已經在台北了?

當你收到這封信時，我已經在台北了
I would have arrived in Taipei when you receive this letter.
I would have been in Taipei the moment when you receive this mail.

這句話是標準的未來完成式運用，其寫法為I will have been in Taipei by the time you receive the letters. 為何receive用現在式而不用未來式(will receive)，因為我們主觀認定會發生，就如The train leaves at 8.火車即將於8點離站，雖然尚未發生仍需用現在式表主觀認定事實。
若是你的問題指的是過去發生，則先發生用過去完成式後發生用過去式以利对照，寫法為 I had been in Taipei by the time you received the letters.
由於你沒明確指出是过去或未來發生，所以我把所有答案都列出供你參考使用，有問題可提出。

(grammar) connected with the form of a verb used to express actions in the past
The past is the time that has gone by(=the letter); things that happened in an earlier time(=in Taipei)=past perfect tense;
When two actions happened in the past one after another, we use the past perfect tense for the first action.
eg:-By the time you received the letter I had been in Taipei (already).
eg:-already=adv.
---I've already arrived---past perfect tense
---When you got the letter earlier, I had already arrived Taipei.
---or::-Had the letter been received, I should have gone to Taipei.---conditional clause + Main Clause.