Impulsivity is one of the main symptoms of ADHD. That means your child might act without any thought to the future consequences, for example, imagine that your son with ADHD is playing with a toy. He sets it down and moves on to something else. His little sister comes along and grabs the toy he is no longer playing with and your son reacts by grabbing it back. He didn't think about your household rules of "no grabbing" and impulsively reacted to the situation. He didn't worry about what the consequences would be.
Children with ADHD have a hard time attaching future consequences with the present moment. To do so requires stopping, thinking through the situation and deciding if the consequences are worth the effort of doing the "right" thing. Many children with ADHD have a difficult time thinking through the situation and connecting something that may or may not happen in the future. This might be partially because of impulsiveness.
Another reason is that children with ADHD are often several years behind emotionally, in other words, a ten year old child with ADHD might be closer to a seven year old in maturity. Because you see your child as ten years old, you have certain expectations for behavior, however, your expectations might be above your child's capabilities.
Finally, children with ADHD often have trouble with working memory. They aren't able to hold relevant information in their brains to use it in the current situation. That's why many children with ADHD seem to not learn from their mistakes.
There are ways you, as a parent, can set more effective consequences and help your child curb impulsiveness.
Be specific when explaining your expectations. Asking your child to "be good" isn't providing him with enough information to know what he should do. Instead, state exactly what your expectations are, such as "no jumping on the furniture" or "sit down and complete the first three math problems." Make sure to make eye contact when stating your expectations and don't assume your child' understands. Have him state back what you said.
Have realistic time limit on consequences. A consequence should be long enough for your child to think about his behavior and short enough that he can move on to positive behaviors. For example, if you use time-outs, you can use one minute per chronological year minus three (to adapt the time-out to your child's maturity age). When consequences are too long, your child is apt to forget the original behavior by the time the punishment ends.
Impose consequences as soon after the behavior as possible. Because children with ADHD have a difficult time connecting future consequences to present behavior, instituting a consequences an hour or more after the behavior probably won't have any affect. Try to make consequences instantaneous with the misbehavior if possible.
Focus on positive behaviors****. Providing praise for behaviors you want rather than punishing those you don't usually works better for children with ADHD. When your child is playing nicely with his sister, take the time to let him know you noticed. As with all children, attention is what drives both good and bad behaviors. If you pay more attention to the unwanted behaviors and less to the wanted behaviors, your child will seek the attention and misbehave. Choose your battles.
Use praise, charts and token economies** to help drive appropriate behaviors.** Young children do well with praise and charts, such as receiving a star for completing his homework, straightening his room or doing something the first time you ask. For older children, offer praise and increase their privileges, if possible, for example, completing and handing in homework all week can result in extra computer or video game time on the weekend.
When your child is misbehaving, you might find you want to just be away from him. But, it is better to find time to spend doing something fun with your child each day. Try to set aside at least 15 minutes a day for each child. You might spend it reading together at the end of the day, playing a game together or going for a walk. All children benefit from one-on-one time with their parents.