Severus Snape
Dolores Umbridge's patronus forms a cat. You find this out in The Deathly Hallows.
A goat. Because he did inappropriate spells on a goat. It also is proven in the Deathly Hallows.
It is not a hologram, but a patronus (spell to defend against Dementors and send messages to other wizards in no time). The deer patronus belongs to Snape, and very specifically to Lilly Potter (Harry's mum). Snape loved her so much that he took over her patronus as a sign when she was killed by Voldemort. The patronus represents itself to Harry and helps him find the real sword of Gryffindor.
Avada Kedavra
There is no book after Deathly Hallows, it is the last.
It is a spell preformed in the Deathly Hallows by Hermione Granger. This spell strengthens an area or enclosure from enemies.
Fred died in the deathly hallows! :((((((
A sheld spell
I highly doubt it. Dementors are only mentioned once or twice in general in Deathly Hallows (the most memorable mention is during the Horcrux search when Harry couldn't conjure a Patronus because of Tom Riddle's Horcrux locket) so the chance they would be in the trailer is very small.
The Deathly Hallows is being divided into two movies. After the Deathly Hallows Part II, the series will be over.
You will need to find the confringo spell and cast it at the barrier
The seventh and final novel in the series is called; Harry Potter and the Deathly Hallows.However, the final book in the series was split into two movies: Harry Potter and the Deathly Hallows Part 1 and Harry Potter and the Deathly Hallows Part 2.