You Can Speak Nicely & Firmly at the Same Time
As my Spiritual Master Teacher Bernice used to say, "You catch more flies with honey than vinegar." Be caring, be nice, be direct; and your message will surely come across.
Speaking angrily projects hot, controlling energy that the people you are talking to can actually feel—most often making them angry too, even if they don't show it at the time—and even if they do know that they made the mistakes.
That's why it's always better to let go of your anger before you correct anyone, not after. Otherwise, all that they feel, and thus remember, is your anger, not your words, and certainly not your good intentions.
Speaking angrily may get a child to comply, especially when you are the child's parent or authority figure; but obedience out of fear will almost always lead to alienation and rebellion over time.
And as many people have already learned, people may not always remember what you said—but they will always remember how you made them feel.
