Deuteronomy 4:1,5-9; Psalm 147:12-13,15-16,19-20; Matthew 5:17-19
Jesus makes reference to Deuteronomy and the other books of the Pentateuch in todays homily to his disciples. Not one jot, not one iota are to be forgotten. These 'diacritics' in the written form of Hebrew, I am told, make a huge difference to the meaning of words: they were of key importance.
But - Jesus has a marked directness of application of all those laws. For example, we must love our neighbour to the same level as love of God (Leviticus 19:18 and Deuteronomy 6:4 in Mathew 9:13). This keeps all of the meaning of the old law, but makes it clear that it all must be applied. One can not pick and choose what one likes to live by!
Note however that an over-legalistic, harsh application of the law is equally wrong. Todays reading is a good example from Deuteronomy explaining how the Law allows us to Love and to be Loved by God.
The Law of the Lord is Perfect - it revives the soul.(Psalm 19).