i'm an expert (in sth.).较幽默/专断的（用不同口气、表情说有不同效果）
or i have done this for thousands times.幽默略自大的
or i can do it when dreaming.幽默略自大的
or my past experience is enough for my qualification to this job/work.较文绉绉的
or you can never find a man who knows more about it than me.幽默
or believe me ,i have enough experience.最基本说法
还有无数种表达.
他为什么没在这里
why isn't he here?最基本,口语
why is he absent?稍文雅
i wonder the reason of his absence.非常文雅
is anyone going to tell me the exuse of his absence here?领导者的口气,可略带质问
