Wishful thinking (idiom)

　　●The imagining or discussion of a very unlikely　　　　 future event or situation as if it were possible and might one day happen.（一廂情願的念頭，妄想）

Examples:　　

　　1.Hoping for a car as a birthday present is just wishful thinking. Your parents can't afford it.　　

　　2.Matthew wanted to be a basketball player, but with his height that was wishful thinking.

　　Duty-bound (to do sth) (adj.)

　　●Legally or morally responsible for doing　　 something. （責無旁貸，義不容辭）

Examples:　　

　　1.We are duty-bound to justify how we spend our funds.　　

　　2.We felt duty-bound to challenge the decision.

Cast a shadow over/on sth (idiom)

　　●To spoil a good situation with something unpleasant.（投下陰影）

Examples:　　

　　1.The tragedy cast a shadow over their lives.　　

　　2.Her father's illness had cast a shadow over the birth of her baby.

　　Mend one's ways (idiom)

　　●To improve one's behaviour after one has been behaving badly for a long time.（改過自新）

Examples:　　

　　1.She let him stay because he promised to mend his ways.　　

　　2.Threatened with suspension, he promised to mend his ways.

　　Turn one's head (idiom)

　　●If something turns someone's head, it has an influence on how that person behaves, especially by making them too proud.（衝昏頭腦）

　　Examples:　　

　　1.Success has never turned her head - she's still the same simple unaffected girl she always was.　　

　　2.Winning that prize has turned his head.