～たら
RuleVerb ta-form + ra: the most versatile conditional, "if/when X (happens)."
Curiosity
Japanese has four conditionals: tara, ba, to, nara. Start with the most all-purpose one, tara.
Intuition
Add ra to the ta-form (past/perfective). Its "once that has happened" nuance fits both hypothesis ("if it rains") and time ("when I arrive").
Visualization
taberu (dictionary) → tabeta (ta-form) → tabetara (conditional). It passes through the ta-form.
食べるdictionary form
Essence
Form it as ta-form + ra (any verb, keeping its sound-change). It covers future hypotheses and past discoveries ("when I opened it, ..."). It can be followed by commands or requests (tsuitara denwa shite), so it is less restricted than to or ba.
Examples
駅に着いたら、電話してください。
When you arrive at the station, please call me.
安かったら、買います。
If it is cheap, I will buy it.
宿題が終わったら、遊べる。
Once the homework is done, I can play.
Mini-quiz
Add the tara-conditional to "nomu" (to drink).