in和with是介词,后面跟名词.in 或with后加名词=形容词加ly,这句话的意思是说：In 或with 后加名词就相当于是一个副词,在句中作状语.例如：
I will give him the present in person.
可变为 I will give him the present personally.
其中in person的意思是“亲自”修饰动词give,修饰动词的词是副词,所以in person就相当于是一个副词.
还有in general就等于generally,
He jumped with joy.可变为
He jumped joyfully.
关键理解这句话In 或with 后加名词就相当于是一个副词.