grammar - How to appropriately pair tenses in subordinate and main clauses?

Consider the following combination. Which is the correct one?

1. テレビが私の国で 《できた》 のは１９６０ 《です》。


2. テレビが私の国で 《できる》 のは１９６０ 《です》。


3. テレビが私の国で 《できた》 のは１９６０ 《でした》。



4. テレビが私の国で 《できる》 のは１９６０ 《でした》。

Note: The important words are intentionally bracketed for the sake of emphasizing.

Answer

In English, the tense of the main clause and relative clauses is usually relative to the time at which the sentence is spoken.

I waited until the bus came.

You use the past tense on both verbs because both the waiting and the coming happened in the past. But while you were waiting, the bus hadn't come yet! So, relative to the action of waiting, the bus coming happened in the future (but still in the past of when the sentence was spoken.)

In Japanese, on the other hand, only the tense of the main clause is relative to now. The tense of relative and subordinate clauses is relative to the time the main clause happens. So in the above sentence, the verb "came" would actually be in the non-past (aka. present) tense, even though it (the bus coming) has already happened in the past:

バスが来るまで待った。

If the whole thing hasn't happened yet, only the final verb must change:

バスが来るまで待つ。
I will wait until the bus comes.

This entire concept is also why 後で always takes the past and 前に always takes the non-past. With 後で, the relative clause is always in the main clause's past (happened before it), and thus must take the past:

食べた後で歯を磨いた。
After eating, I brushed my teeth.

Even if both eating and brushing teeth is in the future, 食べる must be in the past because it is past relative to the main verb (磨く).

The opposite of this happens with 前に. It is always after the main clause- in the main clause's future- so the verb before 前に must be in the non-past form:

歯を磨く前に食べた。
I ate before brushing my teeth.

Even (indirect) quotes follow this rule, as they are subordinate clauses. In fact, this allows the speaker to give a bit more information when compared to English:

私はなおみが学校にいると思った。
私はなおみが学校にいたと思った。
I thought Naomi was at school.

These both translate to the same English sentence, but they do not share the same meaning. The former implies that the speaker thinks Naomi is at school as he's thinking, while the latter means the speaker thinks Naomi was at school sometime even further in the past (but probably somewhere else at the time of thinking). Again, changing only the main verb and retaining the tense of the subordinate verbs still holds these temporal relationships.

This may sound crazy, but in your example, I believe the the relative clause is actually happening at the same time as the main clause. The reason I say this is because both the subordinate and the main clause are referring to the same thing- the creation date, which will be the same from 1960 and forward. But because you're talking after 1960, both 2 and 4 will sound weird, as 1960 is in the past. Option 1 works because it's just stating the current creation date- 1960. Option 3 also works because it's stating the creation date at some point in the past- still 1960. So either option 1 or option 3 will work fine.