With all due respect (介詞短語,多用於句首,引導批評或異議,含義為:恕我直言)
If I may/might say so (條件狀語從句結構,多用於句首,委婉表示批評或異議)
If I may/might be so bold/blunt(條件狀語從句結構,多用於句首,委婉表示批評或異議)
Correct me if I am wrong (祈使句結構,多用於句首,委婉提出意見或異議)
Let's agree to disagree. (習語,表示雙方各持己見,繼續辯論也是徒然,沒有意義)