The 接続助詞（せつぞくじょし） (conjunction particle)
たら is used as a kind of ‘halfway point’ between ‘if’ and ‘when’ in Japanese. It expresses the completion of an (A) action, and then a (B) action that is considered to be the result of (A). It may be hypothetical, or it may be presenting a one-time result of something that happened in the past.
To use
たら, it will replace た in the simple past form of verbs,
い-Adjectives,
な-Adjectives, and nouns.
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今度（こんど）遅（おく）れたら、許（ゆる）さないよ。
When/if you are next late, I will not let it slide.
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寒（さむ）かったら、エアコンつけてね。
When/if you are cold, turn on the air conditioner.
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彼（かれ）の事（こと）が好（す）きだったら、彼（かれ）に言（い）った方（ほう）がいい。
When/if you like him, you should tell him.
-
明日（あした）だったら、遊（あそ）べるよ。
When/if it is tomorrow, I can hang out.
Despite appearing to be the past form of the word that it is attached to,
たら itself does not actually carry the nuance of ‘past’. Other words in the sentence will need to be examined to see whether
たら is truly highlighting something from the past, or simply the completion of something (which may or may not happen in the future).
-
来週（らいしゅう）だったら、いけるよ。
If it is next week, I can go. (Highlighting a future event)
-
先週（せんしゅう）だったら行（い）けたのに。
If it was last week, I could have gone. (Highlighting a past event)
Although
たら is regularly used to mean ‘if’, it almost always has a nuance of ‘when’. This is due to
たら specifically highlighting the following situations:
完了（かんりょう） - Completion of (A).
成立（せいりつ） - Materialization/coming to fruition of (A).
These meanings are opposed to other ‘if’ structures like
ば, and
なら, which focus far more strongly on the hypothetical ‘if’ meaning.
-
買（か）い物（もの）に行（い）ったら、クッキーを買（か）ってきて。
When/if you go shopping, please get me some cookies.
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あなたがその本（ほん）を読（よ）まないなら、私（わたし）が読（よ）みます。
If you are not going to read that book, I will read it.