I wanted to know when you tear your rotator cuff what happens?

You lose the ability: To lift the arm away from the body to the side, ie, abduct the arm. Its painful.

Depends: It really depends on the size and pattern of tear and the cause. Alot of tears are degenerative and age related. Most patients with these will present with pain with overhead motion. Don't necessarily lose ability to lift arm. Large sudden traumatic tears following a fall are often times associated with severe pain and weakness with lateral arm raise or unable to lift arm at all.