〜たら

たら is the most flexible of the four conditionals — it covers both hypothetical if and temporal when, and uniquely allows intention / command / request / invitation in the consequent clause.

家に着いたら、電話する — "When I get home, I'll call." This is a temporal-when reading: the speaker is announcing the intended sequence. Try this with と and it sounds wrong (と would suggest "calling" is a natural consequence of arriving, like a reflex); try it with ば and it sounds stiff. たら fits naturally.

Hypothetical use: もし時間があったら、行く — "If I have time, I'll go." The optional もし ("if perchance") makes the hypothetical reading explicit.

Formation: take the た-form of the verb and add ら. 行く → 行ったら, 食べる → 食べたら. For adjectives: 寒い → 寒かったら, 静か → 静かだったら.

A past-tense たら in the result clause produces the "happened-to-find-out" reading: 家に帰ったら、誰もいなかった — "When I got home, no one was there" (and I discovered this unexpectedly). This is one of the most natural ways to express discovery in Japanese.