You use plainly to indicate that you believe something is obviously true, often when you are trying to convince someone else that it is true.
The judge's conclusion was plainly wrong...
Plainly, a more objective method of description must be adopted...
You use plainly to indicate that something is easily seen, noticed, or recognized.
He was plainly annoyed...
I could plainly see him turning his head to the right and left...
If you say something plainly, you say it in a direct and honest way, without trying to hide the facts.
'You're a coward,' Mark said very plainly and soberly...
Few of our political leaders are willing to talk plainly and honestly about the emergency facing the country.