Dogs use highly expressive body language, and ear position can be a significant indicator of a dog's mood. To read him correctly, study his ears and correlate what they are saying with the appearance of other parts of his body, including his eyes, mouth, tail and general posture.
Ears Relaxed
Learn what your dog's ears look like in a neutral position. The long, droopy ears of the basset hound may not change much, and the large, erect or "prick" ears of a corgi can make him look alert even when he's feeling mellow. Subtle changes in ear position can speak volumes.
Ears Slightly Back
A slightly back position can easily mean more than one thing, so you'll need to check the dog's other indicators. Combined with a relaxed mouth (tongue showing), lowered head and freely wagging tail, this position probably means he's feeling friendly. On the other hand, slightly back ears with a closed mouth and tight lips, lowered head, crouching posture, tucked tail and the whites of the eyes showing (called "whale eye") can mean he's anxious and defensive, and may bite.
Ears Flat Back
A dog with his ears pinned all the way back, clamped tight to his skull, with his eyes wide, his tail tucked and his body low and hindquarters well under him is probably terrified and about to leave the scene at warp speed.
Ears Back and Low
A dog with ears back and held low on his head, with squinty eyes, grinning mouth, low body and a raised paw, is showing submission. If he's just acknowledging a leader, he may lick and whine. If he's submitting out of fear or after a fight, he may even roll onto his back and pee a little. It's his way of saying, "I give up—please don't hurt me!"