"'Should not your piety be your confidence and your blameless ways your hope? Consider now: Who, being innocent, has ever perished? Where were the upright ever destroyed?'" NIV translation
Eliphaz reminds Job that he is a religious man and asks him why he does not take comfort in that fact during these trials. He is sure that Job has a "hidden sin" that God is punishing him for but he tries to assure Job that God will look at all of his religious works and have mercy on him. This same type of attitude is widespread even among Christians today as many think that, if their good works are more than their sins, then God will have mercy on them. We are reminded that God's demonstration of mercy to us is not based on how we follow religious rituals but flows from His very nature. He shows us mercy because He is merciful.
"'As I have observed, those who plow evil and those who sow trouble reap it. At the breath of God they perish; at the blast of his anger they are no more.'" NIV translation
Eliphaz basically tells Job that a man reaps what he sows and is asking him what he is hiding. This was probably a well known saying at the time based on the experiences of men.
Although this is a very sound principle that even Paul referred to in the Bible (see Galatians 6:7 & 8), it does not explain everything. This is forgetting the fact that, as sons of God, we have an enemy that is at work in the world. As we see in Jesus' parable of the weeds (Matthew 13:24 & 25), you can plant good seed and still have weeds among the crop as the devil does his thing. Many times, we can see bad things happening to people and we immediately assume, like Eliphaz, that it is because of some "hidden sin" in their lives. It could be just the opposite as it was in the case of Job where it was happening because of his righteousness.
Eliphaz leans on his experience as he says "As I have observed" but he had not observed things that were happening in the spiritual realm. This is a reminder to us that we have to filter what we see in the physical with what we know of the spiritual and that is only possible through the Holy Spirit that lives in us as Christians.