Matthew 17:9 Jesus told Peter, James, and John not to tell anyone what they had seen until after His resurrection because Jesus knew that they didn't fully understand it and could not explain what they didn't understand. This question revealed their misunderstanding. They knew that Jesus was the Messiah, but they had much more to learn about the significance of His death and resurrection.
Matthew317:10-12 Based on Malachi4:5-6, the teachers of the Old Testament law believed that Elijah must appear before the Messiah would appear. Jesus referred to John the Baptist, not to the Old Testament prophet Elijah. John the Baptist took on Elijah's prophetic role - boldly confronting sin and pointing people to God. Malachi had prophesied that a prophet like Elijah would come (Malachi 4:5).
Matthew 17:17 The disciples had been given the authority to do the healing, but they had not yet learned how to appropriate the power of God. Jesus' frustration is with the unbelieving and unresponsive generation. His disciples were merely a reflection of that attitude in this instance. Jesus' purpose was not to criticize the disciples, but to encourage them to greater faith.
Matthew 17:17-20 The disciples were unable to drive out this demon, and they asked Jesus why. He pointed to their lack of faith. It is the power of God, not our faith, that moves mountains, but faith must be present to do so. The mustard seed was the smallest particle imaginable. Even small or undeveloped faith would have been sufficient. Perhaps the disciples had tried to drive out the demon with their own ability rather than God's. These is great power in even a little faith when God is with us. If we feel weak or powerless as Christians, we should examine our faith, making sure we are trusting not our own abilities to produce results, but in God's.