English for Today by Ganiu Abisoye Bamgbose (Dr GAB)
MANNER or MANNERS?
It’s bad ~manner~ to eat with your mouth wide open (nonstandard).
It’s *bad manners* to eat with your mouth wide open (standard).
If someone has good manners, they are polite and observe social customs. If someone has bad manners, they are impolite and do not observe these customs.
She smiled again in a friendly ~manners~ (nonstandard).
She smiled again in a friendly *manner* (standard).
The manner in which you do something is the way that you do it.
GAB

Thank you, sir