そうしたら

そ

う

し

た

ら

Common wordFreq.FrequencyTop 50,000

Meanings

1

usually written using kana alone

Connects one event to what happened next. Common in speech and informal-to-neutral writing.

そしたら
そしたら is a casual, contracted version of そうしたら with the same 'then' and 'if so' meanings.
そうすると sense 1
そうすると expresses the same 'and then' sequence as そうしたら but often sounds more logical or explanatory, like 'and as a result'.
それならN4
それなら means 'if that's the case' and is used to draw a conclusion from a stated condition, whereas そうしたら can also simply mark a sequence of events.
そうすれば
そうしたら is more colloquial and common in casual speech, while そうすれば sounds slightly more formal and logical.

From そう, meaning 'so; that way', plus したら, the conditional form of する. Literally like 'if it is/did so', extended to 'then' and 'in that case'.