Not quite: if you stage something, it no longer appears in "git diff" and you have to use "git diff --cached" to show it. One of those things which I trip over occasionally.

"git diff --cached" = staging vs. HEAD

"git diff" = workspace vs. staging (but when nothing is staged, staging == HEAD)

(I think?)

No, this slide is correct. git diff (without any arguments) shows you what you haven't staged. In other words, the different between the working folder (which here is called "workspace") and the index.

Whoops, I think you're right.

I skimmed over git diff, as when I use git diff I constantly re-figure out for what it precisely applies again. I just care to remember "it diffs".