I usually see a strange person with bad hair and bleary eyes in my mirror. Now, if the mirror didn't show me that, I would thank it.
In some other exercise, just "Spiegel" alone was "mirrors", but now it's singular ("mirror")?
"Spiegel" doesn't change according to whether it's plural or singular. The only way you can tell is if the pronoun is in front of it (der for singular since it's a masculine noun, die for plural). I'm not sure why "mirrors" couldn't be accepted too, though.