Matthew 5:21-22 - "Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire."
Matthew 5:27-28 - "Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart."
Matthew 5:31-32 - "It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement: But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced." committeth adultery."
Matthew 5:33-34 - "Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: But I say unto you, Swear not at all; neither by heaven; for it is God's throne."
Matthew 5:38-39 - "Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
Matthew 5:43-44 - "Ye have heard that it hath been said, Thou shalt love thy neighbor, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you."
Matthew 11:21-22 - "Woe unto thee, Chorazin! woe unto thee, Bethsaida! for if the mighty works, which were done in you, had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes. But I say unto you, It shall be more tolerable for Tyre and Sidon at the day of judgment, than for you."
Matthew 11:23-24 - "And thou, Capernaum, which art exalted unto heaven, shalt be brought down to hell: for if the mighty works, which have been done in thee, had been done in Sodom, it would have remained until this day. But I say unto you, That it shall be more tolerable for the land of Sodom in the day of judgment, than for thee."
Matthew 12:5-6 - "Or have ye not read in the Law, how that on the Sabbath days the priests in the temple profane the Sabbath, and are blameless? But I say unto you, That in this place is one greater than the temple."
Matthew 12:35-36 - "A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things. But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment."
I don't know about you, but I have often been absolutely sure of my understanding of God's Word and His will, only to find out later that I missed the whole point. I have come to believe that the best way to understand, is to make sure you are "on right terms" with the Lord, that you pray for guidance, and that once you believe you understand, ask yourself how to apply it in your life. Notice I say "your life" - for until you have allowed the Word to change you, I do not recommend you "fix" your brethren! Ezra 7:10 puts it this way:
"For Ezra had prepared his heart to seek the Law of the LORD, AND TO DO IT,
and (then) to teach in Israel (its) statutes and judgments."