"Darkness" is a term for something non-existant, and therefore not an actual thing. Due to this, it has no mass, and so it has no movement, and so it has no speed. However, due to the fact that darkness exists due to light, one might say that darkness has the speed of light if one is talking about how fast shadows can form, because obviously, they are as fast as light is, just like a mirror reflection is exactly as fast as whatever it is reflecting.
The term "building" is also a term. It is not literally the word that describes that something is in the process of being built (if that was the case then why wouldn't you call computers or anything else that is built "buildings"?)
I don't know the science behind crying, so I can't give a definite answer. I'm gonna guess that you can cry underwater.
If ladybird is a type of bird and does not refer to the gender, then I would imagine that this too is a term that is not to be put literally.
Amnesia isn't exactly a disease (although it can be a consequence of diseases), so it's not exactly something that can literally be "cured". However, a lot of people only gain temporary amnesia, and in these cases, the memories return to them eventually.
