When we enforce unreasonable expectations, and especially when we punish according to them, we put stress on kids, who respond by avoiding, escaping, and becoming irritable. Ironically, that puts them off whatever activity, skill, or virtue we're trying to inculcate, making it aversive rather than attractive. So how can a parent seek to counter the natural tendency to expect too much behavior from children?
First, aim to build competencies by inching toward success gradually, and focus on process rather than successful outcome: That is, focus on trying to do what's valuable, not on immediately reaching the level of performance you think a child of that age should reach. If you encounter strong resistance, then back off for a few days, and when you return to the issue, lower your demand. Seek to get the desired behavior for a shorter period, ask for less of it, or take some other step to defuse the all-or-none dynamic. Working up to the desired behavior gradually, in doable steps, is a process called shaping.
For example, let's say your child is lagging behind the rest of his class in reading. His teacher wants you to work with him at home on his reading every day for 20 minutes. Your child, who's embarrassed about his reading, resists this "extra" work, perceiving it as an unfair penalty. The resistance, on top of the reading problems, produces a situation that can make a parent crazy with frustration and anxiety. One move you can make in response is to try something low-key, like, "We're going to read to each other. You read for two minutes, and we'll talk about what you read, then I'll read for two minutes and we'll talk about it." Then, once you've got the habit in place, over a week or two you can escalate in easy stages up to 20 minutes of reading.
Try to bear in mind that you feel your child's resistance to learning to read, or perhaps his genuine difficulty with reading, as pressure on you. Your stress goes up, and, since you're not a saint, it's very likely that your increased stress will translate into behavior (such as harsh categorical statements in your Metallica voice about doing 20 minutes of reading every single day or else) that causes his stress to go up when you try to get him to work on his reading. So it's crucial that you separate the pressure you feel to help your child read from the project of working with him on his reading. If that stress gets into your voice, it affects the process.
Or reconsider what's vital and what's negotiable in your demands. Take the example of the non-napping child. Parents know that a child of that age should take a nap, and they've picked a time of day when that nap should happen, and yet the child cries or wants to play. If you're in that position, recognize that the problem here is in part the expectation. Shifting it to, say, having the child play quietly in her crib at that time will take care of most of what's really at issue: The child needs to rest, and you need a break. A designated number of minutes of actual unconsciousness on her part is probably unnecessary.
If you find yourself saying, "No matter how hard I try and try, I can't make my kid do X ..." or "No matter how hard I try, I can't make my kid understand Y ..." it's usually a clear sign that expectation and enforcing that expectation are a significant part of the problem. Your expectation may in fact accurately address the mean—that is, you may expect a behavior of your 9-year-old that most 9-year-olds can do—but remember the range of human variability and try to structure antecedents (the things you do to encourage a behavior to occur) with room for that variability.
When your child fails to meet a reasonable—specific, clear, flexible—request and it's a one-time occasion, try to let it go if you can. But if the request is not met and it's not a one-time event, then it's time to begin shaping the desired behavior. Start with a lot less than you will eventually settle for: less behavior, for less time, less often. Ten minutes of homework, not the full hour right away; putting the forks on the table, not setting the whole table. Then work up to the desired level. And, once you get close, remember that getting a behavior to occur most of the time, as opposed to every single time, is probably good enough. Exceptions are usually not a problem; they're normal. As is the case with your own efforts to exercise and eat properly, if it's a habit, and if you do the behavior most of the time, that's good enough.