"He has been a part of a secret organisation since his grandfather's time."
"He has been a part of a secret organisation since his grandfather's time."
"He has been a part of ~" 「彼は〜に属してます」
"secret organisation" 「秘密組織」
"since ~" 「〜から」
"his grandfather's time" 「彼のお爺ちゃんの代」
"grandfather's" 「お爺ちゃんの」
"his" 「彼の」代わりに "our" 「私たちの」"your" 「あなたの」なども使えます。
"time" 「代」"era" も言えます。
He's been part of a secret organization since his grandfather's time.
"彼はお爺ちゃんの代から秘密組織に属してます"
- He's been part of a secret organization since his grandfather's time.
でも、これだけだとすこしわかりにくいです。
His family has been part of a secret organization since his grandfather's generation.
彼の家族はお爺ちゃんの代から秘密組織に属してます。
Since his grandfather's generation, his family belongs to a secret organization.
お爺ちゃんの代から、彼の家族は秘密組織に属してます。
オーストラリア 
アメリカ合衆国 
