Berakhot 20

Rav Papa asked Abaye why miracles happened in previous generations but do not happen to them? Women, Canaanite slaves and children are exempt from shema and tefillin but are obligated in prayer, mezuza and blessings after the meal. Why? Why does each case need specifying? Why is it not obvious that they are exempt from time bound commandments and there is no need to specify? Why are women obligated in prayer - is it not a time bound commandment? Is women's obligation to make a blessing after the meal the same as men's in which case they can say it on behalf of a man and help him fulfill his obligation? Erza instituted that one who has a seminal emission cannot learn Torah until he goes to a mikveh. What should he do regarding saying shema and blessing after the meal? If one thinks something in his heart, is it as if he said it?