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
, and nouns.
When/if you are next late, I will not let it slide.
When/if you are cold, turn on the air conditioner.
When/if you like him, you should tell him.
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).
If it is next week, I can go. (Highlighting a future event)
If it was last week, I could have gone. (Highlighting a past event)
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.
When/if you go shopping, please get me some cookies.
If you are not going to read that book, I will read it.