Questions & Answers about Ég skil ykkur ekki.
Why do we use ykkur instead of þið for 'you all'?
The verb skilja (to understand) forces its object into the accusative case. Þið is the basic subject form, but since 'you all' are the ones receiving the action of being understood here, we use the accusative form ykkur.
How would this sentence change if I were only talking to one person?
You would swap the plural ykkur (you all) for the singular accusative pronoun þig (you). The sentence would become: Ég skil þig ekki.
Why does ekki (not) come after ykkur instead of before it?
In Icelandic, pronoun objects like ykkur usually jump in front of ekki. If the object were a regular noun, ekki would normally come first. For example: Ég skil ekki strákinn (I do not understand the boy) versus Ég skil hann ekki (I do not understand him).
What is the dictionary form of the verb skil?
It is skilja (to understand). When paired with the subject ég (I), it loses the ending and is conjugated simply as skil.