YOU SAID:

I went on holiday to Spain with my girlfriend and one of the days she went to the toilet and left her phone in it, we went back shortly after but it must have been stolen

INTO JAPANESE

私はガールフレンドと一緒にスペインに休暇で行きました、そして彼女がトイレに行って彼女の電話をそこに置いたある日、私たちはすぐに戻ったが、それは盗まれたに違いありません

BACK INTO ENGLISH

I went on vacation to Spain with my girlfriend, and one day she went to the bathroom and put her phone there, we returned immediately, but it must have been stolen

INTO JAPANESE

私はガールフレンドと一緒にスペインに休暇で行きました、そしてある日彼女がトイレに行って彼女の電話をそこに置いた、私たちはすぐに戻った、しかしそれは盗まれたに違いない

BACK INTO ENGLISH

I went on vacation to Spain with my girlfriend, and one day she went to the bathroom and put her phone there, we returned immediately, but it must have been stolen