"a convenient excuse for not going"
"a commodious harbor" "a commodious building suitable for conventions"
误 Please come to see us when you are convenient.
正 Please come to see us when it is convenient for you.
析convenient的意思是“使人感到方便的”,不是“(自己)感到方便的”,因此只能用物〔事〕作主语,而不能用人作主语。