I find that Japanese is a weird language for its learners, especially for those who've already learned English, because in English there are two tenses: present and past.

However, in Japanese there are accomplished and unaccomplished tenses, and these look very strange if compared with English. For instance:

わたしは日本に行くとき、ラジオを買いました。

Its English translation is:

When I go to Japan, I bought a radio.

So the English sentence has two different parts which use two unrelated different tenses, so it's obviously logically wrong.

Please look at another example:

私は日本に行ったとき日本製のテレビを買う予定です。

Its English translation is:

When I went to Japan, I decide to buy a TV made in Japan.

And this sentence also sounds strange.