Mitzvah to Know Torah
Everyone is obligated to learn and know the entire Torah shebichsav and sheba'al peh with all their details and particulars, i.e., Tanach, Talmud Bavli and Yerushalmi, Tosefta, Mechilta, Sifra, Sifri, Tur and Beis Yosef, and Shulchan Aruch with its commentaries (שו"ע הרב פ"ב ס"א ).
Although it sounds like a lot and impossible, a person must nevertheless strive for that. If he puts in the time, he can gradually get there. He must try to the best of his ability.
Learning Without Understanding
Torah shebichsav. One who learns Torah shebichsav, even without understanding it, fulfills the mitzvah to learn Torah (שו"ע הרב פ"ב סי"ב ).
Torah sheba'al peh. If one learns Torah sheba'al peh without understanding it, it is not considered learning. The whole purpose of Torah sheba'al peh is to explain Torah shebichsav – if he doesn't understand, Torah was not explained (שו"ת ארץ צבי ח"א סי' כ').
Nevertheless, even if he does not understand Torah sheba'al peh, he must still try to learn it, since le'asid lavo he will understand all the Torah he worked at in this world (שו"ע הרב שם, צל"ח בהקדמה לגבי לימוד אגדות חז"ל ).
Zohar. Merely saying the words of the Zohar, even without understanding, is significant (חיד"א, שו"ת חיים שאל ח"א סי' ע"ה ).
Four Times
When approaching something new, one must learn it four times to understand it well. Otherwise, he will not fully understand it, even if he thinks he does (ס' חיי עולם ח"ב פי"א, קריינא דאגרתא ח"א סי' ט'). If one learned without reviewing four times initially, he did not fulfill the mitzvah of learning Torah (ח"ח הובא בתשובות והנהגות ח"א סי' תקל"ט ).
Review
One who forgets something he learned transgresses the prohibition of pen tishkach (מנחות צ"ט ע"ב ). Chazal explain that the posuk is referring to all of Torah, not just the aseres hadibros (שו"ע הרב פ"ב ס"ד ).
Some count this prohibition as one of the 613 mitzvos (יראים סי' שנ"ט, סמ"ג לאווין י"ג ). The Rambam, who didn't, held that the issur is midivrei sofrim (שו"ע הרב פ"ג קו"א סק"א ).
One who doesn't review, resembles one who plants but does not reap (סנהדרין צ"ט ע"א ). Therefore, each person must review his learning so as not to forget it. Success in learning mainly depends on review, since reviewing develops one's intellectual skills (קריינא דאגרתא ח"א סי' ו').
One should review even at the expense of learning new material, as it says in Avos: "You are not responsible to finish the job, but you are not free to sit idle (הפלאה הקדמה לכתובות אות כ "א ).
However, if one didn't review because he was learning new material, not because he was sitting idly, and forgot some Torah, it's a machlokes if he is oveir on pen tishkach (פסקים ותשובות סי' רמ ו הערה 187).
If one who supports Torah has a choice between supporting a talmid chochom who reviews and one who doesn't but learns more quantitatively, he should give preference to the one who constantly reviews his learning (מו"ר בשו"ת שבט הקהתי ח"ב סי' ער"א ).
A koveia ittim. One who is koveia ittim must also designate time to review his learning so as not to forget what he learned. It is improper for one to learn a daf a day but not review and even forget what he learned within a few days. He must also find time for review.
However, if one is so busy with his livelihood that he does not have time to review all that he learned, or if one forgot due to old age or illness chas veshalom, he is doesn’t transgress pen tishkach (שו"ע הרב ס"ו ועי' תויו"ט אבות פ"ג מ"ח שכתב שאפ "ה מתחייב בנפשו ).
