I just got a new foster dog and he's 11 months old. When he plays with other dogs at the park he grabs onto their legs with his mouth. It's not hard really (a soft bite), but enough that he'll interrupt the other dogs from running. Most dogs do not enjoy this and will run away and hide. Owners of other dogs especially hate this. How can I teach him to play in a more appropriate way?
The simple answer is: you need to reprimand him whenever he grabs the legs of another dog. Please have a look at this related question: What is an effective way to discipline a dog?.
The difficult part is that any kind of reward or punishment is most effective withing a maximum of 2 seconds after the behavior you want to reward or punish. You need to have a direct connection with your dog to be able to reprimand him any second in the dog park. If you start running to him the moment he grabs another dog, you initiate a different behavior (most likely playing) instead of discouraging his previous behavior.
Also important: don't wait with your reprimand until after your dog has already grabbed some legs. The moment you see him leaning towards a dog's legs or opening his snout to grab them, call his name and sternly tell him "NO!". Don't make it a friendly "I'd really appreciate if you didn't do that" but look at him angrily and tell him "no" in a stern voice, so there can be no doubt in your dog's mind that you dislike whatever he's doing right now.
You might need to discourage his behavior in a physical way, too. This never ever means to hit or otherwise hurt your dog. Most dogs are simply so concentrated on their playing and interacting with other dogs that they ignore their owner's vocal commands. By adding a physical contact, you pull him from his previous thoughts and make him notice you.
The solution is a long leash. They come in length of 20 or even 30 meters and can become quite cumbersome. Additionally, if your dog is 20 meters away from you, you still have no chance of reprimanding him in time, even if he's on the leash. So please use a leash of 5 - 10 meters. That gives your dog enough freedom of movement to play with others while keeping him close enough to give you a chance to interact with him at any second. A short pull on the leash at the right time can act as the physical stimulus to pull him from his playing and notice your reprimand.
An alternative is a remote controlled shock or vibrating collar. I strongly discourage the use of electric shock collars as these are much too strong and the electrodes are situated directly at the throat. Most dog owners I know have made good experiences with a simple vibrating collar, and even those vibrations are often too strong for sensitive dogs.
Start by disabling the electric shock (if the collar has one) and configuring the vibration to the lowest level. Then fit the collar to your dog's neck. It must not restrict movement or breathing, but should not sit too loosely. As a rule of thumb, you should be able to easily fit 2 fingers between the neck and the collar.
Then take a walk with your dog or observe his actions at home. Whenever he does something bad, activate the collar's vibration and see how he reacts. It's enough for him to be startled or confused and stop his behavior. That's all you really need. If he ignores the collar, the vibration is too soft, if he yelps or panics, it's too strong.
Keep in mind that activating the collar itself is not the punishment. It's just the physical stimulus to stop the unwanted behavior and make the dog perceptible for your reprimand. You still have to tell him with your voice that you dislike what he's doing.
Some people prefer teaching their dog alternative behavior instead of reprimanding bad behavior. I honestly cannot fathom how you would implement an alternative behavior to grabbing other dog's legs without making your dog not play with other dogs anymore...