Pronounced "hent eye" but with the "t" sound on the "eye" syllable. Just find a could Japanese text book for clarity.
In Japan this word just means pervert. And is never used to incdicate a genre. To indicate the erotic genre they use "etchi" or "ero." And something they write etchi as the letter "H." Hentai never refers to a genre or a naughty picture. Hentai also means transformation, metamorphosis, abnormality, or formation.
The phrase 'hentai video' wouldn't be used in Japan.
Outside of Japan, it refers to both "pervert" and anime with strong sexual themes. Nudity is NOT hentai. Just simple nudity would be etchi. When something sexual is depicted it's the hentai genre.
Japanese usage: "She thinks I'm a hentai!" (Well, if this were said in Japanese...)
English usage: "I got a cable modem so I can download hentai faster!"