Answer by Professional Japanese Teacher
Well done / Good job
The way to say “good job” is “よくできました yoku dekimashita”. This expression is made up of “よく yoku” (good) and “できました dekimashita” (able to do something).
Yoku comes from the adjective よい yoi (good), which is another word for いい ii. We change it to yoku when describing an action that comes after that. Dekimashita is the past tense of the verb できます dekimasu, which means “to be able to do something”. So putting it together, this phrase means that someone is able to do something and has done it well.
If you are talking to your student, you can also say it informally, like よくできたね yoku dekitane. However, do avoid using this expression outside of class, especially on someone who is more senior than you as it may sound very arrogant or condescending.
This is because this expression is often used by teachers praising their students on getting full marks or by parents to their children when they did something good.
So for someone senior, perhaps it’s better to use compliments such as “すごいですね sugoi desune” (that’s amazing) instead.