It all depends on exactly what you are trying to undo/revert. Start out by reading the post in Ube’s link. But to attempt an answer:
Hard reset
git reset --hard [HEAD]
completely remove all staged and unstaged changes to tracked files.
I find myself often using hard resetting, when I’m like “just undo everything like if I had done a complete re-clone from the remote”. In your case, where you just want your repo pristine, this would work.
Clean
git clean [-f]
Remove files that are not tracked.
For removing temporary files, but keep staged and unstaged changes to already tracked files. Most times, I would probably end up making an ignore-rule instead of repeatedly cleaning – e.g. for the bin/obj folders in a C# project, which you would usually want to exclude from your repo to save space, or something like that.
The -f (force) option will also remove files, that are not tracked and are also being ignored by git though ignore-rule. In the case above, with an ignore-rule to never track the bin/obj folders, even though these folders are being ignored by git, using the force-option will remove them from your file system. I’ve sporadically seen a use for this, e.g. when scripting deployment, and you want to clean your code before deploying, zipping or whatever.
Git clean will not touch files, that are already being tracked.
Checkout “dot”
git checkout .
I had actually never seen this notation before reading your post. I’m having a hard time finding documentation for this (maybe someone can help), but from playing around a bit, it looks like it means:
“undo all changes in my working tree”.
I.e. undo unstaged changes in tracked files. It apparently doesn’t touch staged changes and leaves untracked files alone.
Stashing
Some answers mention stashing. As the wording implies, you would probably use stashing when you are in the middle of something (not ready for a commit), and you have to temporarily switch branches or somehow work on another state of your code, later to return to your “messy desk”. I don’t see this applies to your question, but it’s definitely handy.
To sum up
Generally, if you are confident you have committed and maybe pushed to a remote important changes, if you are just playing around or the like, using git reset --hard HEAD followed by git clean -f will definitively cleanse your code to the state, it would be in, had it just been cloned and checked out from a branch. It’s really important to emphasize, that the resetting will also remove staged, but uncommitted changes. It will wipe everything that has not been committed (except untracked files, in which case, use clean).
All the other commands are there to facilitate more complex scenarios, where a granularity of “undoing stuff” is needed 🙂
I feel, your question #1 is covered, but lastly, to conclude on #2: the reason you never found the need to use git reset --hard was that you had never staged anything. Had you staged a change, neither git checkout . nor git clean -f would have reverted that.
Hope this covers.