modify与qualify都有“修饰”的意思，但是modify表示让读者对于其修饰的名词有更深的理解，而qualify是对被修饰名词的限制。

Grammarians talk about modifying a noun. Whenever one uses an adjective that expands how we understand what a person, place or thing is like, we say that adjective modifies that noun.

In the sentence, “The man bit the dog,” any adjective you include that adds to the reader’s understanding of the man or the dog are modifying adjectives. When you modify a noun with an adjective, you add to your readers’ or listeners’ understanding.

On the other hand, when you qualify a noun, you’re giving limitations, you’re adding conditions to that noun so that your readers and listeners can better distinguish it from other nouns of the same class. So when we say, “The German man bit the small dog,” the two adjectives “German” and “small” each modify “man” and “dog” respectively. In the sentence “This German man bit that small dog,” we suggest that we need to distinguish between German men, and I wish to talk about this one. Likewise, there may be more than one small dog in the vicinity, and I wish focus your attention on that particular small dog.

In simplest terms, modifying adjectives increase our understanding of the qualities of a given noun, while qualifying adjectives place limits on our understanding of a given noun. So far, so good?

Sorry if this muddies the waters, but if you make a special effort to emphasize “German” and “small” in our sample sentence (This German man bit that small dog.) then their function shifts from modification to qualification. If you can see how “German” and “small” put limiting conditions on “man” and “dog,” rather than expanding our understanding of them (as they did at first), then it should be easy to see the shift from modification to qualification.

最后发现其实没有多大的区别，个人感觉modify可以完全替代qualify