たら details

write + (past) + if

kaita ra

if; when; once — after the condition completes

meaning: if/when you write

駅えきに着いついたら電話でんわして。

eki ni tsuita ra denwa shite.

Call me if/when you arrive at the station. casual

カーテンを開あけたら、外そとは雪ゆきだった。

kaaten o aketara, soto wa yuki datta.

When I opened the curtains, it was snowing outside. neutral

Examples

if / once — the condition completes first, then the main clause follows

駅えきに着ついたら電話でんわして。

eki ni tsuitara denwa shite.

Call me if/when you arrive at the station. casual

東京とうきょうにきたら、ぜひ連絡れんらくしてください。

toukyou ni kitara, zehi renraku shite kudasai.

Please do get in touch if/when you come to Tokyo. neutral

雨あめが降ふったら、運動会うんどうかいは中止ちゅうしになります。

ame ga futtara, undoukai wa chuushi ni narimasu.

If it rains, the sports day will be called off. neutral

会議かいぎが終おわりましたら、こちらからご連絡れんらくいたします。

kaigi ga owarimashitara, kochira kara go renraku itashimasu.

Once the meeting is over, we will contact you. formal

when — past-tense discovery, did X and unexpectedly found Y

カーテンを開あけたら、外そとは雪ゆきだった。

kaaten o aketara, soto wa yuki datta.

When I opened the curtains, it was snowing outside. neutral

Morphologically た＋ら, so it rides any polite stem too: polite style says 終わりましたら, not ×終わったらです.

ば
ば suits lawlike, repeatable conditions and leans written; たら is the spoken all-rounder and the only one comfortable with one-off sequences — 着いたら電話して cannot become ×着けば電話して.
なら
たら fires only after its condition completes (着いたら＝once you have arrived); なら takes something already said or assumed as its premise, so the main clause may even precede the condition in time — 行くなら早く出ろ.
ても
Both suppose a situation on the same た／て morphology, then split — たら makes the outcome hinge on the condition (降ったら中止), ても concedes the supposition changes nothing (降っても決行); swapping them flips the sentence's logic.

たり〜たりする
One kana apart on the same た-stem — 行ったり samples "doing things like going", 行ったら sets a condition "once (I) go"; the り/ら swap flips a list into a conditional.