根据句意，写出括号内所给单词的适当形式。
小题1:Many of my friends were invited to the party last Sunday because of my _________ ( twenty ) birthday.
小题2:Boys, don’t touch the machines, or you may hurt _________ ( you ).
小题3:The International Union Against Cancer says about seven hundred million children _________ ( breath ) smoke-filled air.
小题4:I can’t understand his (silent) on such an important matter.