The other day, to me great surprise I've learnt from an English native speaker that one must avoid using must in sentences and use should instead which did strike me as odd indeed because I do constantly hear native English speakers say things like:
- Alright, I must go now.
- You must have done this a week ago.
- I'm quite impressed, I must say.
I do know the difference between the two but still, I fail to understand why some people say that must must not be used.
Any valid reasons behind that?