What is "Ervas Av"?
Radak: This is the most severe of the Arayos - "Ervas Avicha" (Vayikra 18:7), i.e. your mother, your father's wife.
Why does it say "Gilah" (singular)? Before and after this, it says "Zimah Asu' and "Temeis ha'Nidah Inu Vach" (plural)!
Radak: This hints to "Amon Hirbah Ashmah" (Divrei ha'Yamim II, 33:23) ? he had Bi'ah with his mother (Sanhedrin 103b). Malbim ? also Yehoyakim did so (Vayikra Rabah 19:6). Kings transgressed most of these Arayos.
What is the grammatical form of "ha'Nidah"?
Radak: It is a noun ? a woman who is [Temei'ah] from Dam Nidah. The same applies to "v'Hotzi'u Es ha'Nidah Min ha'Kodesh" (Divrei ha'Yamim II, 29:5), "ki'Ymei Nidas Devosah" (Vayikra 12:2). Sometimes Nidah is an adjective, e.g. "Nidah Hi."
Why does it say "Inu" regarding Nidah?
