The four conditionals side by side, plus the set phrases built on them: ばいい, ばよかった, たらどう, てよかった.
〜と (conditional) V-dict / V-nai + と / i-adj + と / N・na-adj + だと
Verb (dictionary/nai form) + と. Means 'whenever/if A, then B naturally follows'. Used for natural results, habits and machines, not requests or intentions.
春になると花が咲く。 When spring comes, the flowers bloom.
〜ば V1: e-stem + ば / V2: stem + れば / i-adj: -ければ / N・na-adj: なら
Conditional ば: Group 1 → e-stem + ば, Group 2 → stem + れば, i-adj → ければ, noun/na-adj → なら(ば). Means 'if', focusing on the condition needed for a result.
このボタンを押せばドアが開く。 If you press this button, the door opens.
〜たら V-ta + ら / i-adj: -かったら / N・na-adj: だったら
Verb/adjective (ta-form) + ら. The most general conditional: 'if' or 'when', including unexpected results after doing something.
駅に着いたら電話してください。 When you get to the station, please call me.
〜なら V/i-adj plain + なら / N・na-adj + なら
Plain form + なら. Means 'if that is the case' — responds to what someone said or a known situation, often with advice or a suggestion.
京都に行くなら、秋がいいですよ。 If you're going to Kyoto, autumn is best.