My understanding of A たとき B is that it is used when B happens after A; A るとき B
is used when B happens before A. (Related question: [What is the difference
between ～たとき vs ～るとき](https://japanese.stackexchange.com/questions/19332/what-
is-the-difference-
between-%EF%BD%9E%E3%81%9F%E3%81%A8%E3%81%8D-vs-%EF%BD%9E%E3%82%8B%E3%81%A8%E3%81%8D))

However, my understanding fails me when I face **たい** sentences:

> 来月 日本へ (行く / **行った** ) 時、富士山を 見 **たい** です。

My teacher explained that 行った needs to be used instead of 行く. Why this is the
case? It seems to me that one would want to see Mt. Fuji before going to
Japan. It would be weird to go to Japan first and suddenly want to go to Mt.
Fuji...
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question
grammar tense
るとき or たとき with たい?
First of all the tense in Japanese is different from English, and the verbal
auxiliary た represents past and completion.

As your teacher says, "来月、富士山に行った時、富士山を見たいです" is natural. I think this た
doesn't mean "past" but "completion". This た can be used for a future thing.
For example, 来週の金曜日に、仕事が終わったら、お酒を飲みましょう (Let's drink after work next Friday).
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answer