それならThen, If that's the case

それなら and それでは can both be used at the start of a sentence as transition markers meaning in that case or well then. それなら expresses a response to a specific condition, as in それなら行きません (In that case, I won’t go). それでは can overlap in usage, but it can also mean well then or and with that, often used to wrap up a topic, as in それでは、失礼します (Well then, excuse me), which それなら cannot convey.3

なら follows a phrase or noun to express a condition, as in 田中さんが行かないなら、私も行かない (If Tanaka isn’t going, I won’t go either). The なら of それなら is the same particle, but the phrase それなら is used sentence-initially to refer back to something previously mentioned, as in 行かないの？それなら私も行かない (You’re not going? In that case, I won’t go either).

それなら expresses a response to a condition, as in それなら行きません (In that case, I won’t go). The conditional たら form can also mark conditions at the end of verbs, adjectives, or nouns. それだったら can convey a similar meaning to それなら, but with a slight nuance: それなら assumes the condition will or has happened, while それだったら tends to be more hypothetical.3