Brief Overview OT vs NT
I will try to give it to you in a simple but scriptural way.
Because of the sinful nature of man and esp of those chosen by God- God, through Moses instituted a set of laws which we call the "old testament". These 600 plus laws were designed to keep men until the savior, which they described and told about would come and bring salvation to all that would come to Him His way.
These laws were to be in effect until "all was fulfilled" that is until all of these laws were fully kept (fulfilled) by a man. The old testament tells us who the man that would keep these laws and fulfill them would be. It gives us His name and how to recognize Him. So when this happens, the laws of Moses are fulfilled and no longer have any use to man.
The "new testament" is what we call the scriptures and dipensation of time that follows this Man replacing the laws of Moses.
Under the "new testament" we are given His example to follow and only two commandments which Jesus says CONTAIN ALL the law AND the prophets.
37 Jesus said to him, "'You shall love the LORD your God with all your heart, with all your soul, and with all your mind.' 38 This is the first and great commandment. 39 And the second is like it: 'You shall love your neighbor as yourself.' 40 On these two commandments hang all the Law and the Prophets."
The new testament actually begins After the resurrection of Christ. All the gospels prior to His resurrection are actually under the old testament which is why before His resurrection He refers to the law of Moses. After His resurrection the law of Moses has been fulfilled and no longer is in effect. as He said on the cross "it is finished".
If a Christian keeps any of the laws of Moses he is saying that Christ did not fulfill the law as He said He did and is calling Him a liar.
However we must understand that because the laws of Moses and esp the 10 commands are no longer in effect for us it does not give us license to sin. In fact we are to live a life that is ABOVE what the law requires. Jesus said that He gave us His EXAMPLE to follow and He lived a better life than the law required . And the 2 commands He gave us are all encompassing.
I hope this helps you to understand. The "new Testament " is not a new thing added later, it was originally given to and understood by Abraham way before Moses.
The whole of scripture was planned in advance by God and that is clearly shown in the Bible.
God bless you as you pursue His truths