To give a long speech about what is right or wrong, often in a way that is boring or annoying to others.
Sermonize can make people think of someone talking too much about what they think is right.
Examples:
He likes to sermonize about eating healthy food.
She sermonizes about the importance of recycling.
The teacher sermonizes every day about doing homework.
My dad sermonizes about going to bed early, but I like to stay up late!
The mayor sermonizes about keeping the town clean all the time.
My little brother sermonizes about sharing his toys with me all day long.