Suppose you are a student and want to work part time as a teacher. Then, you have two alternatives: either to teach on one-on-one basis or to teach to a class. Which do you think is more difficult? My guess is that when asked this question, more people will say it is the latter that is harder. But in fact, the former is, or should be the harder.
When teaching to a class, you can decide what to talk about in the class totally according to what you want to place an emphasis on. You can organize your class at will, though you have to choose topics the students will get interested in or of whose importance they will be persuaded. It's totally up to you. Almost anyone can do it on condition that they are given enough time.
On the other hand, when you teach on one-on-one basis, what should be taught is totally dependent on the student. You cannot teach what you want. Even though your student made an error in a question where it is tested whether she correctly understands relative pronouns, it is often the case that your lesson should not be about them, but about other topics such as cases of pronouns. What should be taught should be determined according to the student's misunderstanding that caused the mistake. Moreover, this probing into where the misunderstanding lies is really difficult, for it is often found that the problem have already gotten so complicated that the teacher doesn't know where to start in order to disentangle the Gordian knot.
ゴルディオスの結び目 Gordian knotという表現を最近知ったので、使ってみました。私は単純に「難解な問題」という意味で使っています。生徒の問題がかなり複雑になっていてどうやって解決したらいいのかわからない、と言いたくて、生徒の抱えている問題が複雑になっている様子がこんがらがった紐の結び目のように想像できたので、この表現を使いました。