interested!

I don't know if I am using the negation technique correctly - can someone share what the correct negation of D would look like?

Can someone please explain whether the concept of negation is tied to validity of the argument? I may be misunderstanding but negation doesn't seem like a logical inference so when would I want to employ this tool or what kind of question stem would signal "oh I should negate this" ?

I understand that the contrapositive for conditional statements gives you extra relationships to work with and can help you identify what is missing in an argument and/or link conditional statements together. I understand how it's a tool in the mental toolbox for tracing the line of an argument and making new connections and there may be questions that test whether the contrapositive is valid. But when/how does negation come up?