What is the minimum measure of semen that imparts impurity when touched?
Halacha 1: the minimum measure for one who touches semen is a lentil-sized portion; for one who discharges it, any amount (it exits his body).
Question 2
A woman discharged semen exactly two 12-hour periods (onot) after having intercourse. Is she impure?
Halacha 11: within three onot after intercourse (not counting the onat of the intercourse itself), she is impure like one who had a seminal emission.
Question 3
A man dreams of intercourse, feels the sensation, but finds no visible semen on waking — yet his flesh is warm. Is he impure?
Halacha 5: if he felt the sensation of intercourse in his dream and his flesh is warm, he is impure — the lack of visible semen means it was absorbed into his skin or clothes.
Question 4
Is a Jewish woman who discharged the semen of a gentile (non-Jew) within three onot impure?
Halacha 17: gentile semen is absolutely pure; therefore a Jewish woman who discharged it, even within three onot, is pure.
Question 5
In which cases listed in this chapter is the impurity only Rabbinic (not Torah-level)?
Halacha 15: those who are impure due to dreams, questionable urination, etc. are impure only Rabbinically — applicable to terumah but not ordinary food. Torah-level impurity applies only to actual emission, intercourse, and touching semen.