| Eruvin
The mitzva of Talmud Torah consists of both learning Torah, and knowing Torah. And of the two, it is the former that is more important. One can determine how much time and effort one puts into learning Torah; but how much one actually knows includes factors beyond one's control--first and foremost, the level of intelligence G-d has blessed you with.
When we complete a tractate of Talmud, we recite the hadran. In this beautiful ode to learning, we state that, "we [those who study Torah] work hard and are rewarded, and they [who do not study Torah] work hard and do not receive a reward". On the surface, this statement is patently false, as many who toil in non-Torah fields are amply rewarded. One could argue that this is referring to spiritual rewards; but even here, many fields of endeavor that are not pure Torah do fulfill mitzvoth, and any field that contributes to earning a living is certainly one worthy of a spiritual reward.
However, it is only in the field of Torah where one is rewarded for effort. All that can be demanded from us is our best effort; the rest is up to G-d. In the world outside of Torah, it is only the bottom line that matters. No one really cares how much effort you utilize. All that matters in the "real world" is what you produce.
No better example of effort exists than the case of Rav Preidah. One of his students was what we might call a slow learner, and needed to be taught the same material more than once--many times more than once. The Gemara (Eiruvin 54b) tells us that it required 400 (that is not a typo) repetitions before he fully understood the material.
I am not sure if we are to be more amazed at the unbelievable superhuman patience of the teacher, or the remarkable diligence of the student. And lest one think that this student was not paying attention the first 390 times or so, the Gemara tells a remarkable story. One day, Rav Preidah informed his student he would have to leave early and the student decided that there was no point in paying attention, as there would be no opportunity to go over it the customary 400 times. As is wont to happen, it so happened that Rav Preidah did not have to leave quite that early, and he managed to teach the lesson 400 times. However, as his student was daydreaming much of the time, he did not understand it. Rav Preidah then taught the student another 400 times. A heavenly voice asked if Rav Preidah would prefer to be granted an extra 400 years to his life, or if he would prefer that he and all his generation be granted the World to Come. When Rav Preidah answered the latter, G-d said: give him both.
Apparently, the dedication of one relatively unknown Talmudic Sage was enough to bring his entire generation to the World to Come. I am reminded (and if anyone knows a source, please let me know) of the story--told of, if I am not mistaken, Rav Yisroel Salanter--who was asked why he must learn so many hours a day. He answered that if he learned less, the yeshiva student might then learn 12 hours instead of 15, the pious layman might learn 3 nights a week instead of 5, the Jew in Poland may then stop observing Shabbat, and the Jew in Paris may intermarry. Just like a food chain, there is a learning chain; and one who is determined to ensure that others understand Torah is king of the Jewish people.
Rav Preidah's name is most instructive. The world preidah means to separate, a seemingly strange name for someone who would not leave his student. Perhaps his name is meant to suggest that when entrusted with a task--and there is no greater task than to teach others--we must not leave that task until we succeed. Doing so can bring great rewards to an entire generation.