One night,I saw a strange man wearing a mask and holding a knife so I kept on stearing at him.I saw that he robbed a lady of her bag and ran away.The lady called the police and the man was arrested at last.
“rob somebody of something”的意思是从某人那里抢了什么东西,而“rob a lady's bag”的意思是抢了一个女士的包（但那个包不一定是从女士手上抢的）,所以,应该说robbed a lady of her bag 而不是robbed a lady
's bag.