Could I speak with the manager, please?
可以叫你们经理过来吗？

Could we have a doggie bag?
可以给我们一个打包袋吗？

Can I get that to go?
可以外带吗？

There seems to be a mistake.
账单有点问题。

I didn't order this.
我没点这个

1. What's up?
注意在打招呼的情景下他们不是真的想问你你咋的了，就是打个招呼而已。。

2. How's it going?
可以理解成问:你最近咋样了?

3. What's going on?
可以类似理解成问你:最近如何?

4. What are you up to recently?
这个通常用于你跟对方想多聊几句的时候这么问

5. How are you doing those days?
要是你好久没看见某个人了，就这么关心问候一下

回答的时候也要分情况，如果你们只是短暂的相遇，你可以稍简洁点说：
Great / Pretty Good 还不错
Same old / Can't complain / It's alright 还行吧，凑合
以后终于不用how are you? fine, thank you, and you了吧~~

1. Not exactly 不完全。

2. I guess so 可能是吧。