He never cries for no reason so when he does cry, he must have really hurt himself.
He very rarely cries so when he does cry, it must really hurt.
「たいしたことではいつも泣かないから、泣く時は、そうとう痛いんだね」= He never cries for no reason so when he does cry, he must have really hurt himself. / He very rarely cries so when he does cry, it must really hurt.
※ 女の子の場合heの代わりにsheを使います^^
ボキャブラリー
never cries for no reason = 理由なしで泣かない
when he does cry = 泣くときは (doesを使うと「泣く事」を強調する)
must have ~ = ~に違いない、~だろう
really hurt himself = そうとう痛い
very rarely cries = 滅多に泣かない
He doesn't cry over small things even if he falls, so when he does cry, it must really hurt.
この場合、次のような言い方ができますよ。
ーHe doesn't cry over small things even if he falls, so when he does cry, it must really hurt.
「彼は転んでもちょとのことでは泣かないので、泣く時は本当に痛いんだと思う」
He doesn't cry over small things. で「彼はちょっとのことでは泣かない」=「大したことでは泣かない」
ご参考まで!