What I am? (medium)

Question: I live with first ones ,they use me well! For the others I am hell! For the third ones I am useless,you can't touch me but you can see me! I am always there within you! Seek for me and and I'm right there! What am I?

Question: Act in my name, and thus remove me. What am I?

Question: I work in a battle of wits. The battle is again with only two while an army each has. Each must take a turn in stratagem but to win the leader must die. What am I?

Question: A van with 15 seats carried only 14 passengers plus the driver. Each passenger entered the van alone and occupied only one seat. Without considering the heart of the driver, upon critical observation, there are 29 hearts beating in the van. How is this possible?

What Am I? (medium)

Question: Sometimes, I leave you with “mixed” feelings. More times than not, I teach you a lesson. I give what I didn’t create. What am I?