Your dog is 9 months old and considerably bigger than they were. They grab your sleeve halfway through the walk, and they mouth your hands when you get home. It hurts now in a way it did not at 4 months old.
You are probably worried this is the beginning of something. In most adolescent dogs it is the same puppy behaviour, in a stronger jaw, at a higher level of excitement.
What this usually looks like
Excitement mouthing comes with a loose, wriggling body, a wagging tail, bouncing front feet, and a dog who is trying to get closer to you. The teeth are part of the game rather than a warning.
Safety note
A stiff, still body, a hard stare, a growl or a lifted lip is a different thing. So is a dog who is trying to make you go away. Stop training and read Get help now. The same applies to any bite that broke skin.
Three things to stop today
Stop pulling your hand or sleeve away fast. Movement is the reward, so a snatched hand pays the grab. Go still and boring instead, then feed a treat off the floor.
Stop the wrestling and hand games entirely for 2 weeks, including the ones you enjoy. You cannot teach two rules about hands at the same time.
Stop meeting them at the door with talk and touch when they are already over the top. Say nothing, scatter 6 treats on the floor, and let sniffing take the edge off first.
Tonight, 6 minutes, before dinner
Hold a treat in a closed fist at your dog's chest height. Wait, saying nothing. The moment their mouth comes off your hand, open it and feed.
Repeat 8 times. You are paying the mouth-off moment, which is the behaviour you want at the door and on the walk.
Finish with 30 seconds of chewing something legal and long-lasting, so the session ends with the mouth busy and the body settling.
Reward-based training is what the veterinary behaviour guidance recommends, and reward-based training has been associated with fewer reported problem behaviours. Relaxation is also trained as a behaviour in its own right, in short sets.
The leash grabbing specifically
Leash biting usually starts where the walk gets exciting, or where it stalls. Think of the first 100 feet, a corner near the park, or a pause while you talk to someone.
Carry a soft tug toy clipped to your belt. When your dog goes for the leash, present the toy and let them win a 10 second tug.
Then walk on. Give the toy 3 or 4 planned times per walk before your dog asks, at the places where they usually grab.
If the grabbing is happening in the first minute, do 5 minutes of sniffing in the yard before you leave the house.
Staying below the point where your dog can no longer take food from your hand is the condition for learning outdoors. A dog who is spinning and grabbing is above that point.
What not to do
You will be told to hold their muzzle shut, pin them, or spray something in their mouth. A review of aversive training methods found them associated with worse welfare outcomes and no training advantage.
Dogs trained with aversive methods showed more stress-related behaviour than dogs trained with rewards. Around a set of teeth near your hands, that is the last direction to go.
Tonight's version, in one line: closed fist, 8 reps, pay the moment the mouth comes off.