puppies and cats

(Should I have put this in the training forum? I thought it might be better here since I accidentally went into so much detail about other pets and stuff... sorry about all the cat stuff, but it is important because it explains my experience with pets, and my pets experience with other pets, and...

My puppy and my cat have been sort of on and off the last couple of days. This morning, the puppy was very gentle with the cat. This evening, though, he tackled the cat. He also latched onto the cat's tail. The cat doesn't seem all that disturbed, but it did hurt him. What can I do? I am...