i bought a present for my mother on her birthday.
I bought a present for my mum on her birthday.
I bought my mum a present on her birthday.
buy sb sth=buy sth for sb 为某人买某物
I bought my mother a gift on her birthday.
I bought a present for my mother on her birthday.
I bought my mother a present/gift on the day of her birthday.
I bought my mom a gift on her birthday