I am assuming that you would like the work A Modern Instance to be explored due to realism and naturalism. As a result I will use A Modern Instance as an example of how William Dean Howells impacted realism and naturalism as a whole.
First, let's define realism and naturalism so that we can see its effects in A Modern Instance. Realism in literature, of course, shows itself in a presentation of things "as they are" in the real world. No idealism can be involved at all. Naturalism takes realism to the extreme in that naturalism uses the detail of realism in order to depict certain social conditions or environments that shape humanity. Therefore, as you can see, the difference between realism and naturalism is one of "degree of intensity."
In A Modern Instance we can see both realism and naturalism. The title comes from "the modern instance" of the marriage (or should I say the DIVORCE) of two characters: Marcia Gaylord and Bartley Hubbard. All of the realistic details in the story revolve around one question:
If judgment must be based on any human activity, should the judgment be based on the motive which prompted the action or on the consequences of the activity?
The detailed activity, of course, is divorce. However, all of the small details in the novel revert to this question. For example, when Bartley actually gives Bird a concussion, Bartley tries to defend himself only to be one-upped by the doctor saying that "Intentions have very little to do with physical effects." When Marcia and family travel to Indiania for the divorce legalities, they are going to protest against this wrong done to Marcia. Their point (and especially the point of the Squire) is that people simply act and that the consequences happen automatically. Other characters attest that Marcia is to blame for the divorce by "setting a course of events in action." Still others, like Clara, think that there cannot be any justice because of Marcia's good intentions. Clara is strictly told the following:
Oh, the meaning doesn’t count. It’s our deeds that judge us.
As a result we can see, as readers, the vivid modern details that make this a realistic novel and how it approaches naturalism because of details due to the "social conditions" of marriage and divorce. There are so MANY details, in fact, from so MANY characters that the reader may tend to get a bit overwhelmed.
This was really the first incidence of divorce dealt with in a modern novel. The idea of divorce as a "social condition" of humanity in naturalism was completely new at this time. Thus, A Modern Instance is an important example of both realism and naturalism in that regard.