Parents and teachers make many requests of children. These requests are assumed to be reasonable (by the adult), but children may refuse to comply. Children may have many reasons for refusing to comply with a request, and understanding the reasons is usually helpful in getting compliance.
Can't do or won't do?
The first question we should ask is if a child is refusing to do the task because they "can't do" or if they simply"won't do?" If a child is incapable or believes that she is incapable of doing the task, then it is a "can't do" problem. We usually address this by teaching and practicing with the child to make it easier. Teaching and practicing a task that the child is refusing to do, however, can be difficult. Having some kind of a reward as an incentive can encourage them to put forth the effort and "give it a try." Giving them feedback as they get better can help them become more motivated and more confident (which makes it easier to get compliance in the future).
A "won't do" problem means that the child is able to do the task, but it is unpleasant for them or they would rather do something else. It's very common for children to want to do something else (for example, playing or watching TV), and a task like "clean your room" is naturally not appealing. One approach for getting compliance in this case is to allow the child to do what they want to do after they finish what you've asked them to do. When needed, you can also offer bigger rewards (for bigger tasks). This is essentially like paying your child for doing the task, which is not necessarily a bad thing (they will learn about that in the future anyway). Many parents are afraid that their child will come to expect a reward for "every little thing", but this rarely appears to happen and we can always reduce how often they get the rewards in the future. It is important to not think of it as "bribing" the child, though, because bribes are given for unethical or inappropriate activities, whereas rewards are given for appropriate activities.
Avoiding direct conflict (aka "opposition")
There are many things we cannot make a child do, and children will eventually discover this during direct conflicts with a parent. It does not help to get into a "power struggle" with a child because we 1) give them something they can oppose, and 2) they will sometimes win. By setting up choices for a child, we reduce the opportunity for them to directly oppose a parent, and essentially leave it up to them. If they want to play a favorite game, then they need to first complete their chore. If they refuse to complete their chore, then they simply do not get to play their favorite game. This is less about fighting with the parent (though expect the child to whine and nag; see Tantrums) and allows the child to make the decision for herself.
Providing multiple choices can be helpful at times as well. We may tell a child that she can choose which chore she wants to do and give her two or three options. It is usually not recommended to give too many options (which can be overwhelming). When a child selects one of the options, then she may be more willing because it was "her choice" (even though you set the options).
Why should I?
Always keep in mind that it is a fair question for the child to ask, "why should I?" when you ask them to do something. Just because it's a "fair question" doesn't necessarily mean that you have to engage them in some kind of debate or negotiation. However, acknowledging to yourself or anticipating this question can make it easier to understand the child's resistance and how to best overcome that resistance. There is further discussion about working with resistance and defiance in this post about "making concessions."
The last point I would like to make is: try to remain calm and patient. Defiance can be extremely frustrating, but becoming angry is not a good long-term solution even if it gets an immediate response. Your child watches and learns from how you handle these situations.
