It is prohibited to study Torah on Tisha B'av, since Torah bring joy to a person, as it says "Pikudeh Hashem Yesharim Mesamche Lev." Only subject matter pertaining to mourning and the destruction of the Bet Hamikdash may be learned. The Rema (OC 553) takes this one step further and records a custom that even on Ereb Tisha B'av, from midday, it is prohibited to learn Torah. Moreover, he says that even when Ereb Tisha B'av is on Shabbat, this restriction applies. This is somewhat surprising because on Ereb Tisha B'av, and certainly on Shabbat, none of the other restrictions of mourning apply.
However, the Mishna Berura (R. Yisrael Meir Kagan, 1838-1933, Poland) cites the Vilna Gaon that this custom is questionable even on Ereb Tisha B'av that falls on a weekday. Based on this, Hacham Bension rules that if it is difficult for a person to deviate from his normal course of study, and doing so may jeopardize his ability to learn, he may continue to learn his regular subject matters. All the more so this year, when Ereb Tisha B'av is on Shabbat, a person may engage in his regular course of study.
SUMMARY
On Ereb Tisha B'av, a person may learn his regular course of Torah study, if it is difficult for him to engage only in Tisha B'av subjects.
