[quote]
When Snape went to the gate where Harry and Tonks was .......
2 questions

1. He said he wanted to see her patronus for himself.
Q. How did he know it changed?
WE don't know why Snape answered the door- maybe it was to mark him as the doublecrosser or that he is the halfblood prince that the title is about as a clue- Snape in COS also found Harry and Ron when he was outof bounds Snape doesn't like harry and this proves he doesn't like Tonks either making fun of her.

2. Before entering the Great Hall he told Harry,"No cloak".
Q. Why did he let harry keep it, he could have taken it from Harry?
Steal in front of DD when he is there and DD gave the cloak to Harry - no way. What power does a non-head of house have over Harry only in class does snape have power - c COS.