Correct verb tenses for if-would-type sentences

Question

I've been confused about this ever since I started to learn Japanese: What is the correct verb tense for sentences like "If I had known ... then I would have"?

Concrete example:

If I had known that (it was not going to be so hot today) I would've joined the competition.

My attempt:

それを分かったら、大会に出る。

I've been stuck for words in this situation lots of times.

Answer 1

I think you could say:

(もし)それが分かっていたら/いれば、大会に出たのに。
～～、大会に出ていたのに。
～～、大会に出ていただろう(に)。
～～、大会に出たんだけど(なぁ)。
～～、大会に出ていたんだけど(なぁ)。
etc.

Example:

(もし)彼女の電話番号を知ってたら/れば、電話したのに。/してたのに。/してただろう(に)。/したんだけど(なぁ)。etc.
(If I had known her phone number, I would have called her.)

Compare:

(もし)彼女の電話番号を知ってれば/たら、電話するのに。/するだろう(に)。/するんだけど(なぁ)。etc.
(If I knew her phone number, I would call her.)

Answer 2

Since you are talking about things past, you should use past tense.

それを分かっていたなら、大会に出ていた。
or
それが分かっていたら、大会に出ていた。

If you say "それが分かったら、大会に出る。" (Quick grammar fix を -> が) it sounds like you are talking about the future. Which means you haven't know it (it's going to be so hot or not) but you will know it later. And you will decide to join the competition depending on it.

** Description on the quick grammar fix **

I fixed grammar それを ->　それが, but one of my answer is それを... Here is the reason why.

First off, the full sentences of my two answers are:
私がそれを分かっていたなら、大会に出ていた。
それが私に分かっていたら、大会に出ていた。

The two sentences have almost the same meaning, but the nuance is different. The subject of the first sentence is 私, so the sentence focus on 私 weather or not know about それ (it's going to be so hot or not). Which has nuance that speaker (it is equals to 私 in this case) know someone who know about　それ. That means, 私 didn't know what the weather going to be before the competition, but now 私 knows.

The subject of the second sentence is それ. It has extra nuance that 私 might has other information (like where the competition held, how many people are joining it, etc) but それ (it's going to be hot or not) is the important to make decision.

Now, get down to your phrase. The full sentence of "それを分かったら、大会に出る。" is:
私がそれを分かったら、大会に出る。
Since 私 is a subject, it sounds like speaker (the speaker is equal to 私 in this case) know someone who know about それ. If so, 私 usually don't speak like that. Maybe "それを聞いてから決めます。"　(I'll make decision after I heard it.)

The full sentence of それが分かったら、大会に出る。 is:
それが私に分かったら、大会に出る。
Which has extra nuance you know some other information, but what you need make decision to join competition is それ.