It is very important that when we give parental instruction we not only tell the child what to do but we also include the moral reason 'why'.
Unless a child internalises the reason why they must behave in a certain manner they will only behave appropriately when there is someone around to keep reminding them what to do. It is important to raise a child who will be responsible and behave correctly when the parents are not there to remind. In the mind of a child, today's "no" without moral principle is only for today.
A child not only needs to know how to do right but needs to know why. If a child understands the reason behind your request for a certain behaviour they can learn to apply it to any new situation they find themselves in.
In this way the child is taught to think morally as well as act morally.
By the age of three, most instructions should be tied to a moral or practical reason why. If your child tells a lie you could quote Proverbs 6:16-17 “… the Lord hates...a lying tongue” or Col 3:9 “Do not lie to one another”'.
Moral training is often more successful when given in times of non-conflict. This is when children are most teachable. At these times it is important to stress and teach the virtues you want your child to learn, not just the vices you wish them to avoid. Teach the child what good behaviour looks like. Do not only say, "do not hit your sister" but rather, "be kind to your sister".
When working with a child go to the root issue. Consider what virtues you want your child to exhibit and look at the behaviours they are displaying presently. Instead of always correcting a wrong behaviour, guide the child to the behaviour you want them to exhibit. Elevate virtue above vice.