First I'd like to thank the guys at https://unsplash.com/ for this breathtakingly beautiful image of these perfectly metaphoric (cloudy) stairs. Imagining yourself on these mystified stairs. Do you see yourself? Are you standing or are you walking? Are you coming down or are you going up? Or try to imagine any other person. As is always the case in reality we cannot seperate anything from its context, from its environment. Nothing can exist in singularity, alone for itself solely. The context of a nominative agent is action, including the environment and sometimes a second, third entity or even more.
Case is a category that can be only applied to nouns, noun phrases (we will get to that), or pronouns (words like he, she, it). Case has to do with nouns, very simple, but important to keep in mind. Verbs don't have case.
The verb is determined by the Nominativ, i.e. the subject case. If the Nominativ is singular then the verb will be singular, too, this is also true for the plural: The child plays. (Singular) Das Kind spielt. The children are playing. Die Kinder spielen. (Plural, the verb is also in the plural.) So technically, case is related to nouns, i.e. case is nominal. Nominativ and Akkusativ are nominal. What does this imply? Let's say that a noun is anything that you can imagine. The primary object you envision will be semantically translated into the nominative case or Nominativ. Then there is a context.
If you generalize German syntax down to the deepest level of abstraction, you will find two types of sentences that serve to understand the significance and functioning of case.
This sentence pattern is the simplest construction: It contains a nominative value (nominative agent it could also be called) and a verb expressing an action pertaining to this value.
Type II sentence pattern boils down to a nominative value, a verb and an object (in the accusative case) that is affected by this action.
The Akkusativ signifies that there is a movement towards this object and that it is affected by the nominative value (Nominativ). The accusative noun is being affected or "touched" by the nominative noun. You have to interpret "touch" in a very broad sense, it simply means that the accusative object is being affected by the nominative value, often in such a way that an object is changed:
We can depict it by an example. Coffee roasters make coffee. What do they do with raw coffee?
So think of coffee when you think of the accusative. This is the case that signifies that something is being
affected and this mostly to the
effect of change. "They" is the
Coffee is the direct object, we also call it accusative case or Akkusativobjekt.
Sie rösten Kaffee. (They roast coffee.)