Aside from the five basic activities forbidden on Tisha B’Av (eating and drinking, washing, anointing, wearing leather shoes, and sexual relations), the baraita teaches that all of the restrictions that apply to someone who is in mourning for a close relative are applicable to all Jews on Tisha B’Av. A person is not allowed to learn Torah – neither Tanakh nor the Oral Law – except for things that are in the spirit of the day, like kinot, Sefer Iyov and the parts of Yirmiyahu that describe the destruction of the Temple.
The Tanna Kamma also permits learning Torah in an area of study to which one is not accustomed, something that Rabbi Yehuda forbids. Rashi suggests that such study might be permitted because the person will feel frustrated that he does not understand the material, an emotion that fits in with the sentiment of the day. The Rivan suggests that such study involves strenuous effort, which is, therefore, not pleasurable.
Another mitzva that is not mentioned here, but is the subject of some discussion, is whether people put on tefillin on the day of Tisha B’Av. A mourner does not don tefillin on the first day of his mourning since they are referred to as “finery,” and a mourner should not go out wearing such an ornament. The Ra’avad believed that this rule applied to Tisha B’Av as well and ruled that tefillin should not be worn. The Ritva and the Meiri point out that usually burial does not take place on the same day as the person’s death (which would create a situation where the mourner is obligated on a Torah level and would therefore not wear tefillin), so the mourner invariably will put on tefillin later in the day, after the burial is over. There are different traditions today; most Sefardim don tefillin, while most Ashkenazim put them on only after midday, during the afternoon Minha service.