Explaining Tense in Japanese

Question

I find that Japanese is a weird language for its learners, especially for those who've already learned English. In English one usually uses two tenses: present and past, however in Japanese there are accomplished and unaccomplished tenses, and these look strange if literally translated to English. For instance:

わたしは日本に行くとき、ラジオを買いました。
Literally "When I go to Japan, I bought a radio."

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

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

And this sentence also sounds strange.

Can anyone explain how to think about Japanese tenses for sentences like these? I'm really puzzled by it.

Answer 1

Using the present tense in the subordinate clause:

私は日本に行くとき、ラジオを買います。

This would mean 'I (will) buy a radio when I am going to Japan'. You'll probably buy the radio right before you fly to Japan. The past tense for this would be:

私は日本に行くとき、ラジオを買いました。

This means something like 'I bought a radio when I was going to Japan.' Note that the tense doesn't change in the subordinate clause (行く, not 行った).　

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Using the past tense in the subordinate clause:

私は日本に行ったとき日本製のテレビを買う予定です。

This would mean that you'll go to Japan and buy a TV there. The action 'buy a TV' happens after the action 'go to Japan', therefore you use the past form 行った. To convert the whole sentence to the past tense:

私は日本に行ったとき日本製のテレビを買いました。

This is like 'I bought a TV when I went to Japan' (You bought the TV after you arrived in Japan). You don't change the tense in the subordinate clause (行った), regardless of the tense of the verb in the main clause (買う or 買った).