1. In the loop （在消息群内）
"Loop" is like a circle. So, if you are "in the loop", you're part of a group of people who know what's happening.
2. In two minds (犹豫不决）
You're not sure what to do. You feel like doing this, then you feel like doing that, so you are in two minds. It means you're undecided or unsure.
3. In hot water (有麻烦）
It's not good to be in hot water. In English, if you're in hot water it means you are in trouble. So it's not a good thing.
4.In no time（立刻 很快）
In no time" means very quickly.
5. On a mission (下决心）
Means you are determined to do something
6.On my own (独自一人）
What it means is just that person alone; not with any help or not with anyone else there.
7. On good tems with (与某人关系好）
It means a good relationship.
8. On second tought (进一步考虑后，仔细考虑后）
It means on thinking a little bit more.
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