at this moment和in this moment 有什么区别?该怎么用?

at the moment 的意思是“现在”“此刻”,常与一般现在时或现在完成时连用.例如：
I have got enough to do at the moment.
但是,at the moment 有时也作“当时”讲,用于过去时态.例如：
I didn't buy that book,because I had no money on me at the moment.
at this (that) moment 的意思是“此时”、“当时”、“那时”,可与现在时、过去时连用.例如：
Parents may feel sad at this moment.
for a moment 是指“片刻”、“一会儿”,表示持续的时间.例如：
For a moment she just stood there,unable to believe what had just happened.
in a moment 的意思是“很快”、“立即”、“马上”.例如：
I'll be ready in a moment.
a moment ago,刚才,表示刚才发生的事情.
I saw Jim a a moment ago.