〜たら
If… / after… then…machine translation
Formationた-form + ら (verb: 行ったら; い-adjective: 安かったら; noun・な-adjective: 暇だったら)
たら is the most versatile conditional in Japanese: "if A then B" for a specific, one-time event. It's very easy to form — conjugate into the た-form and add ら, and it works with verbs, adjectives, and nouns alike (雨だったら).
Its biggest strength: the second clause is completely unrestricted. Commands, requests, invitations, and intentions are all allowed — 着いたら、電話してください (when you arrive, call me). This is exactly where と and ば are forbidden, so when torn between the four conditionals, choosing たら is usually the safest.
たら also means "after": when the first clause is certain to happen, 〜たら means "when A is done, B" — 家に帰ったら、すぐ寝ます (once I get home, I'll go straight to bed). At that point it's no longer "if" — it's simply about time sequence.
Compared with the other three patterns: と is for inevitable, automatic results; ば emphasizes a general hypothetical condition; and なら reverses the time order entirely — 京都へ行ったら、お土産を買います means buying souvenirs after arriving in Kyoto, while 京都へ行くなら、新幹線がいいですよ is advice about something that hasn't happened yet.
Examples
雨が降ったら、出かけません。
If it rains, I won't go out.
駅に着いたら、電話してください。
When you arrive at the station, please call me.
明日暇だったら、遊びに来ませんか。
If you're free tomorrow, why don't you come over?