You shall not desire your neighbor's house. You shall not desire your neighbor's wife, nor his man-servant, nor his maid-servant, nor his cattle, nor anything that is your neighbor's.
Briefly stated, this means that we should fear and love God that we may not craftily seek to get our neighbor's inheritance or house, nor obtain it by show of right, but help and be of service to him in keeping it. Likewise, we should fear and love God that we may not estrange, force, or entice away from our neighbor his wife, servants, or cattle, but urge them to stay and do their duty.

These two commandments are not specifically referring to immorality or theft because those sins are forbidden by other commandments. The purpose of these commandments is to condemn and forbid the desires, thoughts, and scheming that leads to violation of the other commandments. If the act is evil, the desire to commit it is likewise evil. God is not fooled by legal technicalities! If it is a sin for you to take your neighbor's house, then it is also a sin for you to devise some way to get title to it against his will. Just because you can get your neighbor's property legally, does not make it right. Far too often we hear of two people who both divorce their own spouse so they can marry each other. Such behavior is an abomination in the sight of God, and should never be tolerated by a Christian congregation. If it is a sin for you to have another man's wife, then it is a sin for you to urge another man's wife to divorce her husband and marry you. God has called Christians to a higher standard -- one man, one woman, one lifetime.
We are not only forbidden to steal, we are forbidden to alienate anything from our neighbor. It does not matter that you can do it legally. It does not matter that the world sees nothing wrong with what you do. If you do injury to your neighbor and gain that which is his against his will you have transgressed the law of God. We should never attempt by some subterfuge to get our neighbor's spouse, land, inheritance, or possessions.

Because we are by nature sinful, it is natural for us to seek to acquire as much as we can while giving little thought to anyone else. As a result men devise elaborate means and ingeniously contrive schemes for getting ahead at someone else's expense. Yet, in spite of such dishonesty we pretend to be godly, adorn ourselves finely, and conceal our rascality. Often these schemes are derived from the laws. Lawyers and others assist in this legalized robbery by stretching and twisting the law to make it say whatever suits their case, irrespective of what is right. At the same time, those who are most cunning in perverting the law are regarded as experts.
Therefore, this last commandment is not directed at those who are rogues, but at those who are most pious in the eyes of the world. It is directed at those who wish to be praised and regarded as honest and upright people. Because such people have not openly transgressed the other commandments, they are often blind to their sin and need of God's mercy. This commandment makes it clear that outward civil obedience is not enough. The law of God requires a pure heart.
In far too many court cases, the aim is to get something from one's neighbor, or force him out of his own property. In some cases, people quarrel about a large inheritance. In other cases, they quarrel over property lines. Whatever the reason, they all carefully adorn their arguments in order to make it look like the law is on their side. Likewise, those who use laws to advance their own selfish interests are not open and honest about why they want new laws passed. Instead they hide their true motives and seek to influence legislation through gifts and campaign contributions. What is that other than an underhanded attempt to subvert justice and the rule of law. Yet such men walk around with their heads high, as though they were honest and upright citizens, and no man dares to accuse them publicly of being dishonest.
People are continually trying to slip something over on someone else. They knowingly sell defective merchandise without letting the other person know that it is defective. They think nothing of buying an antique from a widow without telling her that it is worth much more. They also think nothing of taking advantage of those who are really hard up either because they cannot find work, have been injured and cannot work, or for some other reason. Gambling is far too common. Credit card companies are all too eager to offer desperate people money at a high rate of interest, which only makes their situation worse. To rationalize such ungodly behavior they fall back on such cliches as, "We all need to look out for number one." Who could even begin to think of all the dishonesty one could justify by such a pretext? Yet the world does not recognize such dealings as evil, and there are no laws to punish them.
Because the Pharisees allowed divorce for almost any reason, a man who was attracted to another man's wife could devise some way of turning the woman's husband against her and causing him to divorce her. We see one example of this in the fact that king Herod (who wanted to be thought of as an honest and pious man) was married to his brother's wife while his brother was still living. We can understand why such behavior was common before the time of Christ, however, it should never take place among Christians, for Christ has forbidden divorce for any reason other than fornication.
Although such things happen, it should be perfectly clear that God does not want them to happen. We should never deprive our neighbor of anything that belongs to him, nor should we want what is his. Even if you could get what is his in a way that was perfectly legal and respectable in the eyes of the world and go on your way as if you had done no wrong, you have nevertheless injured your neighbor. If such injury is not called stealing and cheating by the world, it is still called coveting. It is wrong because you are trying to get possession of his property against his will, and are unwilling to see him enjoy what God has given to him. If the judge, and everyone else, will not condemn you or require that you return it, God will condemn you. God can see the deceit and malice in your heart. Whenever that deceit is yielded to it leads in the end to open wickedness, violence, and ultimately to hell.

 Therefore, we accept what these commandments plainly say. We should not desire anything that would damage our neighbor or allow and assist in his damage, but gladly leave him what he has, wish him well, and help him to keep it. In other words, treat him as you would like to be treated. Put out of your heart all greed, envy, and any desire to profit at someone else's expense. God wants to remove every cause and source of injury to our neighbor. Therefore He expresses it in plain words, "Thou shalt not covet," etc., for He wants your heart to be pure. Although we shall never attain perfect purity in this world, this commandment, like the others, will constantly accuse us and show us how greatly we need His mercy and forgiveness!

[The above is based upon, and closely follows, Martin Luther's explanation of the Ninth and Tenth Commandments.]