What Am I Riddles

Question: You struggle to regain me.
When I'm lost, you struggle to obtain me.
I pass no matter your will,
but I'm your slave to kill.
What am I?

Question: 500 is at my end and my start, yet 5 is at my heart.
The first letter and the first number make me complete.
My name is that of a king.

What am I?

Question: It takes one word to separate them; otherwise they are inseparable.

What are they?

Question: I have four wings, but cannot fly.
I never laugh and never cry;
On the same spot I'm always found, toiling away with a little sound.

What am I?

Question: What always goes to bed with its shoes on?