If you had brought along your smartphone at that time, what would you have been doing then?
If you had your smartphone with you then, what do you think you would have been doing?
ご質問ありがとうございます。
① "If you had brought along your smartphone at that time,"＝「もし君がスマホをそのときに持ってきていたら」
"what would you have been doing then?"＝「そのときに何をしていた？」
② "If you had your smartphone with you then,"＝「そのときにスマホを持っていたら」
"what do you think you would have been doing?"＝「そのとき君は何をしていたと思う？」
ご参考に。
こんにちは。
さまざまな言い方ができると思いますが、下記のような英語表現はいかがでしょうか：
What would you have done if you had your phone?
携帯を持ってたら何してた？
if you had your phone で「もし携帯を持っていたら」と言えます。
ぜひ参考にしてください。
アメリカ合衆国 
