assure 强调“向保证，使确信” 常用于assure sb of sth 或assure sb that 结构中。
They were assure that everything possible was being done.已经向他们保证，正在尽一切努力
ensure 强调“确保某事发生”，常用于ensure sth,或ensure(sb)that结构中
Registration ensures delivery of mail.挂号寄邮件保证会送到。
insure 主要的意思是“保险，投保”
The old man's house is insured against fire.那位老人的房子投保了火险
reassure强调“安慰忧虑不安的人，使其安心，放心，恢复自信”常用于 reassure sb(about sth),reassure sb that 结构中
The captain reassured the passengers about the strength of the ship.船长向乘客们保证船很坚固，让他们安心。
motionless和sttationary区分
追答stationary 和 motionless都有不動的, 靜止的意思, 都可指肢體的靜止.
但 stationary除了not able to be moved之外,
還有其它的意思: showing no change
例1: The doctors said his condition was stationary.
以及: tending to remain in one place的意思.
motionless主要指肢體上的不動.
例: She sat motionless with horror. 她驚恐萬狀, 一動不動地坐著。