Answer:
I begin your sentences. What am I?
I tell people to be careful when handling boxes.
People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?
What is full of holes but still holds water?
The 8 of us go forth and not back to protect our king from a foe's attack. What am I?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.