问题一、

用to表“对某人有用”；用for表“适合某用途”。英国著名语言学家Frederick·T·Wood所著的《英语介词习语词典》中就有这样的说明：

useful to a person, and for a purpose对某人有用的和适合于某一用途的。例如：

Your suggestions have been most useful to me.你的建议对我极为有用。

Those old dusters will come in useful for polishing the car.那些旧掸子拿来扫汽车倒挺合用。

问题二、

read a story to her和read a story for her都正确，但意义不同：用to读给某人听；用for替（为）某人读。如：

She read her poems to the students.她向学生们朗读她写的诗。

He returned the menu to the waiter near him, "Would you please read this menu for me?"他把菜单还给身边的侍者说：“请您替我读一下这份菜单，好不好？”