Question

for a moment,at a moment, in a moment的区别是什么？谢谢

Answer 1

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。
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.

Answer 2

for a moment
片刻, 一会儿
He might have popped in for a moment, since he was passing this way.
既然他从这里经过, 说不定他会突然来呆一会儿的。

at the moment
此刻, 现在, 目前
The police arrived right at the moment of the explosion.
警察就在爆炸的那个时候赶到了。
I was writing a letter at the moment when he came in.
他进来时我正在写信。

in a moment
一会儿, 立刻, 马上
I really must be getting home in a moment.
我真的必须立即回家。
It was all over in a few moments.
不一会儿就都完了。