Okay, let's play a game to understand the adverbial case. Imagine you are going on a big adventure and you need to know how to get to different places. You have a map that tells you where to go and how to get there.
Now, let's say that the words on the map are like the words in a sentence. You have a special type of word on the map that tells you how to get to different places. These words are like little helpers that show you the direction you need to go in.
In grammar, we call these special helper words adverbs. Sometimes, we need to make them even more special by changing them a bit. This is called the adverbial case.
So, let's take the adverb 'near' as an example. If we say "I walk near the river," the word 'near' is telling us where to walk. But if we change it to the adverbial case, it becomes 'nearly.' So now, if we say "I nearly walk into the river," the word 'nearly' is telling us how close we were to the river.
In summary, the adverbial case is when we change an adverb to show how it's related to the action in the sentence. It's like a special helper that gives us more information about how things are happening.