I consider, that you commit an error. I can prove it.
I apologise that, I can help nothing. But it is assured, that you will find the correct decision. Do not despair.
I consider, that you are mistaken. I can prove it.
In my opinion you are not right. I am assured. Write to me in PM, we will talk.
In my opinion it is obvious. I will not begin to speak this theme.
I think, that you are not right. I am assured. I can defend the position.