Must teach until students understand reasons

Shulchan Aruch(Y. D. 246:10): If a when a teacher gives a lessen and the students don't understand he should not get angry at them. Instead he should patiently teach over and over against until they understand the depth of the matter. On the other hand a student should not say that he understands when he doesn't.  Instead he should continue asking questions until he understands it. And if the teacher gets angry with him he should reply, "This is Torah and I have to learn it but my mental abilities are limited."

Eiruvin(54b):Rabbi Akiva asked, How do we know that a teacher is obligated to keep teaching his student until he has mastered the subject? We learn it from the verse (Devarim 31:19), And you shall teach it to the children of Israel.And how do we know that it must be taught until the students are totally fluent in the material? Because that verse says, “Put it in their mouths.” And how we know that the teacher needs to explain the reasons to the students [and not just teach it didacticly – Rashi]? Because the verse (Shemos 21:1) says, Now these are the laws which You have put before them.

Eiruvin (54b): R. Pereda had a pupil whom he taught his lesson four hundred times before the latter could master it. On a certain day having been requested to attend to a religious matter he taught him as usual but the pupil could not master the subject. ‘What’, the Master asked: ‘is the matter to-day?’ — ‘From the moment’, the other replied. ‘the Master was told that there was a religious matter to be attended to I could not concentrate my thoughts, for at every moment I imagined, now the Master will get up or now the Master will get up’. ‘Give me your attention’, the Master said, ‘and I will teach you again’, and so he taught him another four hundred times. A bath kol issued forth asking him, ‘Do you prefer that four hundred years shall be added to your life or that you and your generation shall be privileged to have a share in the world to come?’ — ‘That’, he replied. ‘I and my generation shall be privileged to have a share in the world to come’. ‘Give him both’, said the Holy One, blessed be He.