The Gemara in Taanis 30a says that learning Torah on Tisha b’av is in the main forbidden, since this is a joyful activity. However a beraisa states two significant qualifications. One is permitted to study topics which sadden rather than provide joy to a person; studying from Iyov, Ekha and the somber prophecies of Yirmiya are permissible. In-depth study should be avoided as Torah learning represents the highest state of happiness (pekudei Hashem yesharim mesamchei lev – see Tehillim 19)
According to R. Yehuda’s position, one is allowed to study unfamiliar sections of Torah. He reasons that, given the unfamiliarity with these segments and the initial difficulty in comprehension, no tangible pleasure will be received (we certainly acknowledge this analysis: Despite the incomparable relish we experience when we
finally master a topic or text, we all undergo an initial struggle in grappling with that which initially appears unyielding). As such, studying in this manner will not produce delight and isn’t disallowed.