What is forbidden in the tenth commandment?

Answer

The tenth commandment forbids all discontentment with our own estate (), envying or grieving at the good of our neighbour (), and all inordinate emotions and affections to anything that is his ().

Read the 3 proof texts (KJV)

1 Corinthians 10:10: Neither murmur 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.
Colossians 3:5: Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry: