You already explained "님" is honorific expression.
"그 선생은 좋아" is also a proper sentence. If you are older than "그 선생" or of the same age, you don't need to add "~님".
But sometimes people add "~님" to show respect, even though "그 선생" is younger or of the same age.
The most safe way is adding "~님" regardless of age.