Q: What is forbidden in the tenth commandment?
A: The tenth commandment forbiddeth all discontentment with our own estate, envying or grieving at the good of our neighbour, and all inordinate motions and affections to anything that is his.

1 Kings 21:4. And Ahab came into his house heavy and displeased because of the word which Naboth the Jezreelite had spoken to him; for he had said, I will not give thee the inheritance of my fathers: and he laid him down upon his bed, and turned away his face, and would eat not bread.
Esther 5:13. Yet all this availeth me nothing, so long as I see Mordecai the Jew sitting at the kings' gate.
1 Corinthians 10:10. Neither murmer ye, as some of them also murmured, and were destroyed of the destroyer.
Galatians 5:26. Let us not be desirous of vain glory, provoking one another, envying one another.
James 3:14,16. But if ye have bitter envying and strife in your hearts, glory not, and lie not against the truth . . . For where enying and strife is, there is confusion, and every evil work.
Romans 7:7. What shall we say then? Is the law sin? God forbid. Nay, I had not known sin but by the law: for I had not known lust except the law had said, Thou shalt not covet. 8But sin, taking occasion by the commandment, wrought in me all manner of concupiscence. For without the law sin was dead.
Romans 13:9. For this, thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love they neighbour as thyself.
Deuteronomy 5:21. Neither shalt thou desire thy neighbour's wife, neither shalt thou covet thy neighbour's house, his field, or his man-servant, or his maid-servant, his ox, or his ass, or any thing that is thy neighbour's.
Colossians 3:5. Mortify therefore your members which are upon the earth: fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry.

Westminster Project - See John Flavel's explanation of the the tenth commandment