Answer: Our Sages (Ta’anit 7a) expound that one is liable for a punishment of death for studying Torah alone, without a study partner. Our Sages add that doing so causes one to become foolish and sin.
It seems that according to the letter of the law, one may not study Torah alone in a steady manner, for Torah study is a complex matter with several different ways to explain every single topic. Therefore, one who studies alone may, very easily, err; this error will thereby be ingrained in him, thus causing him to erroneously interpret the entire Torah and cause others to blunder as well. Thus, when studying Torah in a steady fashion, one must do so with a friend who will prevent the individual from making a mistake.
Nevertheless, the commentaries explain that nowadays, when everyone has a vast array of books which explain every Torah topic in several ways, the prohibition to learn alone no longer applies, for even if one errs in the interpretation of a given topic, one will come to realize his mistake by studying these books. One’s books become the “partners” of those who learn them and delve deeply within them. Indeed, Maran Rabbeinu Ovadia Yosef zt”l would say that one who studies Torah at night, even if one does not have a partner to study with, there is no prohibition involved here whatsoever, since nowadays everyone delves into the many books we have available and these books are our rabbis and study partners (see end of Halichot Olam, Volume 8). Hagaon Harav Chaim Palagi writes similarly in his Sefer Ginzei Chaim.
Hagaon Ya’abetz (Rabbeinu Yaakov Emdin ben Tzvi, son of the famous Chacham Tzvi) adds in the introduction of his Siddur that this prohibition does not apply to those studying Torah in Israel, for the very air of Eretz Yisrael enlightens those who learn Torah in it. In the merit of the special added wisdom of those dwelling in Israel, they are permitted to study alone.
Nevertheless, although Torah scholars who are well-versed in knowledge of Torah are permitted to study alone, this is not correct for everyone, for any person in the beginning stages of Torah study who learns alone is especially susceptible to making a mistake. It is for this reason that Maran Harav zt”l would encourage youngsters not to study alone, for one could easily err in one’s study; he told them to study alone only sparingly.
This is especially applicable to laymen who establish set times for Torah study. Their wives should therefore encourage them to go and study Torah with a G-d-fearing Torah scholar who will guide them along the correct path of Torah and Mitzvot. In this merit, they shall be rewarded with much goodness in this world and the Next.