How to say “When I opened the box at home, it had already broken, so I can't give it to my sister for her birthday.” in Japanese
In Japanese
家で箱を開けたときには、もう割れていました。それで、妹の誕生日にあげられません。
Ie de hako o aketa toki ni wa, mō warete imashita. Sorede, imōto no tanjōbi ni ageraremasen.