Gramatically, the phrase "a box of candles" is correct. Also, if you had more than one box, you would say "boxes of candles."
There would be 400 candles in the 1KG box
Yes
The sentence "There are five pens in the box" is grammatically correct.
Yes, things are on hold is grammatically correct.
Yes.
It is neither. Actually, it is: how old am I?
400
No,it is not grammatically correct.
yes
depends did some1 ask you
It harnesses advanced AI technology
"looking forward to your marriage" is correct.