何时用pack何时用 pack up?

何时用pack何时用 pack up? pack up 用英语单词pack造句 pack build up pack up pack up 造句 造几个英文句子用dress up turn up set off took up get offlook up pack 英语中 pack 和 pack（sth ）up 区别? pack away 和 pack up有区别否? pack it up 请帮我造句 pack up是词组吗?什么意思 weekend何时用单数何时用复数?