I have an impression that students of Japanese sometimes find the Japanese na-adjective "majime(na)" difficult to understand. Both dictionaries and textbooks use "serious" as the English translation, and I feel people learning Japanese tend to think of it as having a negative connotation, like "overly serious" or "uninteresting".
But actually, the word "majime(na)" for "serious" is not negative. The meaning is more admirable like, "diligent," reliable," or "conscientious." So, when I teach "majime(na)," I explain it as written below:
A "majimena" person studies hard, works hard, and protects his or her family. A "majimena" person doesn't gamble too much nor drink from the morning. Japanese women want to marry a "majimena" man."
If the student is a woman, she will chuckle at this. If the student is a man, he will be like, "Oh?", straighten up a little, and begin to study diligently. It's a nice feeling seeing this.