I find a lot of applications really annoying, because they add a lot of features and lock some useful ones behind a paywall. I don't know, but it feels really annoying to download an app only to uninstall it.
Not sure if I ever used the phrase I don't know correctly? Are the above sentences correct?
Yes, correct. I don't know, but . .
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Yes, correct.
I don't know, but . . . . marks this as somewhat informal speech.