She must be at least forty.请问其中的at least是副词,还是介词短语,用来修饰哪个词?

at least是介词短语,相当于副词,它与about,over,nearly一样,可以作程度副词用
关键的是,forty不是名词,而是表示年龄的数词.
如:
She must be at least forty,
She is about forty.
She must be over forty.
She is under forty.
She is at most forty