Byzantium

What is the narrative of the film and how is this revealed in the trailer?

In the beginning of the trailer you see the girl write in her diary about her life which she can't tell anybody. After she writes in her diary just in case anybody finds it, she rips out the pages and throws the pages into the wind. It also says that her mother has been living on human blood for two centuries, giving away the fact that shes a vampire. You can see that the girl in the trailer is turning slowly from human to a vampire as you can see she notices this old man in what looks like a hospital, and then her face expression starts turning guilty, her nail starts to grow really long. Its a cut shot but later on it continues, and you see the girl drag her nail across the elderly mans neck.

There is a girl and a mother relationship so they are really close, but when the girl gets a boyfriend, and tells him about "their little secret". Meaning the mother and the girl being a vampire. So in the end the girl comes into this room her nail grows really wrong and she says "i'm sorry for what im about to do", So we don't know if shes about to do something to either her mother or her boyfriend leaving the audience in a cliffhanger. 

What is the genre (or genres) of the trailer and how do we know?

This trailer/ movie is a hybrid, as it is a mixture of romance/fantasy/action and adventure. We can tell its a romance movie as there's a scene of the girl and her boyfriend kissing.

It's also a fantasy and we can tell this is the obvious genre, as we all know that vampires aren't real. 

There's a bit of action and adventure in the trailer as you see a lot of action in the movie.