You can never fullfill the law, you will fail.You are right of course. They were told to 'follow the Law' exactly because they COULDN'T!!!!
This was SUPPOSED to make them look for Another answer to luife's problem, but instead, in their pride, they thought they WERE following it. That's why Jesus showed them that it was there hearts INTENT, NOT what they actually carried out, they was their undoing.
NIV Matthew 5:20-48
20. For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven. (WOW! Tough act to follow!)
21. "You have heard that it was said to the people long ago, `Do not murder, and anyone who murders will be subject to judgment.'
22. But I tell you that anyone who is angry with his brother will be subject to judgment. Again, anyone who says to his brother, `Raca, ' is answerable to the Sanhedrin. But anyone who says, `You fool!' will be in danger of the fire of hell.
23. "Therefore, if you are offering your gift at the altar and there remember that your brother has something against you,
24. leave your gift there in front of the altar. First go and be reconciled to your brother; then come and offer your gift.
25. "Settle matters quickly with your adversary who is taking you to court. Do it while you are still with him on the way, or he may hand you over to the judge, and the judge may hand you over to the officer, and you may be thrown into prison.
26. I tell you the truth, you will not get out until you have paid the last penny.
27. "You have heard that it was said, `Do not commit adultery.'
28. But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
This Law is a standard for human conduct, a perfect standard. Because it was perfect, and we are not, it is impossible for sinful people to keep. It was for this reason that the Law became a stumbling block. It became an obstacle to Man because it is an unattainable perfect standard. The Law, then, brings about the opposite of what it requires. The Law says to be perfect, but shows you where you are not. It says to be holy but condemns you when you are not. Since it is not possible for us to keep the Law and therefore earn our position with God, we then need the holiness of God given to us -- because there simply isn’t any way for us to attain to the standard of God. Therefore, "...the law was put in charge to lead us to Christ that we might be justified by faith," (Gal. 3:24). That is, the Law shows us that we can’t get to God by what we do. We need the grace of God in Christ Jesus manifested in His sacrifice.