Hot answers tagged tense
It's not prohibited, but it never ever means what past tense in English does. Tense in Japanese subordinate clause is (basically) relative to main clause, so if you bought a book and the book was heavy, you just have to say 重い本を買いました. 重かった本を買いました suggests the book was heavy before you bought it. But there rarely are books that being sometimes heavy, ...
This sentence is unnatural. "I bought a book that was heavy" is translated as "重い本を買いました". Sequence of tenses isn't necessary in Japanese. For example, "I read an interesting book yesterday" is translated as "私は昨日、面白い本を読みました".
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).
The past-tense ～た時 pattern is used when the action is completed relative to the main clause. The present-tense ～る時 pattern is used when the action has not yet been completed relative to the main clause. A common example to illustrate the difference: 日本に行くとき、カメラを買った。 On the way to Japan, I bought a camera. (The action of "going" isn't complete yet.) ...
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