Yevamos 84a-85a
There are circumstances when the marriage was permitted yet the levirate marriage is prohibited \u2022 There are circumstances when the levirate marriage is permitted yet the initial marriage was prohibited \u2022 There are circumstances where both are permitted, and where both prohibited \u2022 A Kohenes may marry a Chalal, a disqualified Kohen\u2022 A Kohen should try to marry a Kohenes \u2022 Men and women are typically subject to the same prohibitions, with three exceptions