correct
the box was heavy although i lefted it
المطلوب تصليح although

but or however because although comes before the main sentence . but and however comes before the contrast\

although تاتي بعدها سبب التناقض

but or however

but is the correct answer

I lifted it.Although the box was heavy,

but instead of although is the most suitable answer

The box was heavy,but I lifted it

but or however
The unexpected action comes after but or however
and before although

but is the tru answer

so هى الحل يامعلمين

لان الترك كان نتيجة للسبب

although the box was heavy , i lifted it
or
i lifted the box although it was heavy

اخي وحبيبي واستاذي ناصر لطفي الكلمة هي lift بمعني يحمل وليس left بمعني يترك اما ان تقصد مهبف بمعني غادر فهذه الكلمة تاتي مع fog او mist فقط
جزاكم الله خيرا والحل الصحيح هو but او however او yet
1- the box was heavy , yet ilifted it
2-the box was heavy .However, i lifted it
3- The box was heavy ,but I lifted it.
ملاحظة
انتبه من تصريف الفعل فالماضي منه لابدا ان يكون ed
اخوكم هلال العربي مدرس بمدارس النخبة الاهلية بالرياض

plz , could you tell me what lefted means

and I would like to tell you that most of us are qualified and experienced teacher so it is no use playing tricks on us . if a respected teacher wants to add some useful pieces of information , he says it directly and polietly

sorry i type fast , experienced teachers
he says them