这四个词组都是由moment构成的介词词组，其意思和用法如下：

(1)at the moment “此刻，那时”，常用于一般现在时、一般过去时和进行时。

eg．I’m busy at the moment.

我这会儿很忙。

I didn’t buy that book，because I had no money on me at the moment．

我没买那本书，因为当时我身上没带钱。

Tom is having breakfast at the moment．

汤姆此时正在吃早饭。

(2)for the moment暂时，目前。

eg．We had to stay in the Inn for the moment， as the hotel around are full．

因为周围的旅馆都住满了，我们只好暂住小客栈。

(3)for a moment“一会儿，片刻”，表示片刻的延缓，常与延续性动词连用。

eg．I had to think for a moment before I remember his name．

我想了一会儿才记起他的名字。

(4)in a moment “立即，马上”，指在片刻时间内或后．多与将来时或过去时连用。

eg．The film will start in a moment．

电影马上就要开演了。

It was all done in a moment．

马上就全做完了。