I'm quite aware of how you must feel. 我很清楚你(在这种情况下)的感受. Must本来是必须的意思,比如 You must do your homework now!你必须现在做功课! You must follow the instructions to...你必须按照指令去. 但在这个句子里must有结合因果关系表示应该,肯定的意思.比如说知道某人的亲人去世后,你会说,I know you must feel awful now.意思就是:我知道你现在一定很难过吧~