Training your children is a pretty simple concept when you follow how kids learn naturally. Whether it's dropping a spoon from the high chair and watching to see if it hits the floor, or banging the glass shower door to see if it makes a noise, kids will test cause and effect. The most common area of testing - you guessed it - is testing their parents to see how they 'work'!
Your child is wondering:
"When I whine, do I get what I want?"
"When I throw a tantrum, will I get what I want quickly?"
"When I snatch, do I end up with the toy?"
"When I ask nicely, do people listen to me?"
So - teaching them is kind of obvious. When you see or hear behaviour that you want repeated - reward it with a positive answer if possible. "Mom, would you mind if I had a little more sugar on my cereal?" Hmm, tempting to say, "No, you don't need it." That might be the right response if you're teaching them about health at the moment. But if you've been training your children about asking nicely, perhaps a "Yes" would help your lesson.
When you see behaviour that you do NOT want to see repeated, DON'T reward it. A tantrum must NEVER be an effective way to get what you want. Whining must always be corrected and repeated in a more pleasant tone of voice before the request is granted.
Make it a priority to stop and listen when your child is speaking in a polite tone of voice. It is often easy to ignore that, because the gentle tone of voice does not demand immediate attention. But it is of utmost importance that gentle voices are rewarded with answers, not with being ignored. If you realise your child is needing to repeat a gentle request and that you haven't yet answered, turn and look at them, down on their level if they're little, and say into their eyes, "I'm so sorry, my love. Thank you for speaking nicely to me and being patient. How can I help you?"
When you treat your children with respect, you will be training your children to respect you, too.