when it comes to ...; where ... is concerned
Follows a noun and introduces a topic, often implying that the situation naturally leads to a certain result or characteristic.
彼となると、話が長くなる。
When it comes to him, the conversation gets long.
日本の夏となると、やはり花火大会ですね。
When it comes to Japanese summer, it's definitely the fireworks festivals.
if it turns out that ...; in that case
Follows a clause and presents a hypothetical or realized condition, often with a sense of 'if that's the situation'.
明日雨となると、試合は中止だ。
If it turns out to be rainy tomorrow, the game will be canceled.
彼が来ないとなると、計画を変えなければ。
If it's the case that he isn't coming, we'll have to change the plan.
となると is common in everyday conversation, whereas とあれば is more formal or literary in tone.
となったら sets up a specific hypothetical scenario ('if/when it comes to that'), while となると more often marks a natural consequence or topic shift ('given that...').
になると neutrally describes a change of state or topic ('when it becomes X'), while となると adds a stronger logical or hypothetical nuance ('if it comes down to X, then...').
Derived from the conditional form of だ (である) combined with the particle と, literally 'if it becomes that'. The exact historical development is uncertain, but it functions as a fixed expression.