要马上 表示很快，所以要用soon
later on， shortly later都表示稍后，而不是马上
这句话用英文翻译成：
I'm going to leave soon.
这里的soon就是马上的意思。
be going to是情态动词，加上动词原形后，表示说话人计划或打算做某事，如：
I'm going to paint my house white.
Are they going to have a party on Christmas Eve?
I'm going to be a grandmother in July so I'm going to stay in Switzerland for a month just to be with my daughter.
I'm about to set off
最好是be about to 马上去做某事
later on, 稍后
shortly later ,不久以后
soon,,很快
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