Berakhot 21

From where do we derive that one needs to make a blessing after eating and before learning Torah? Can we say that the reverse also applies? The gemara brings various cases where one is unsure if one already said shema or the blessing after shema or shmone esreh? What does one do? Is shema a Torah obligation or from the rabbis? If one says the everyday blessing on Shabbat, does one continue or stop immediately? If one remebers one has already davened when one is in the middle of a blessing of shmone esreh, does one stop in the middle or finish? If one enters a shul and has already prayed, does one pray again with them? If one entrers shul and they are already saying shmone esreh, does one wait until they finish kedusha or modim, or does one start one’s own shmone esreh – on what does it depend? Does one say kedusha as part of their silent shmone esreh? Does one answer to kadish if one is in the middle of shmone esreh? Rabbi Yehuda says that one who has a seminal emission can says blessings of shema – it seems to imply one can also learn Torah. How can that be? A contradition to Rabbi Yehuda is brought from a mishna further on. How is it resolved?