How to undo changes?

[Lin Mac <mkl0301@gmail.com>]

Hi,

1.
I have multiple files modified, but would like to undo the changes in
one of the changed files.
"git reset --hard" do not accept single files. So I used to use "git
diff <file> | patch -p X", but X cannot be know until I checked the
patch and know the path relationship. Is there other way to do it?

2.
Sometimes I delete old unused branches with "git branch -D <branch>",
but I just realized theren't a reflog log for such operation. If the
branch is the only reference to the logs, deleting the branch makes
the commits become dangling commits.  How could I recover those
commit?

I run into situation 1 from time to time. Luckily I have never ran
into situation 2, but would like to know the recovery ways.
Thanks in advance.

Best Regards,
Mac Lin