Now verse 20 is very strange in some ways. Jesus has told His disciples to pray that the events will not take place in winter, now He says that God has already decided to shorten the days of the trouble. This again should make us beware of any simplistic understandings of predestination and the sovereignty of God. God is sovereign and we should pray and work for good results. Both these things are true.
Jesus warns the disciples to be on their guard against false prophets and messiahs. In times of trouble we are tempted to turn to any one seeming to offer a solution. Some of these false prophets and messiahs will even do signs and wonders and will be very deceptive. Sings and wonders were part of the marks of an apostle and were a feature of the early church. They should still be a part of the life of the church today, but we also need a sound knowledge of the Scriptures and to personally know the Lord Jesus. This is the only thing that is guaranteed to guard us against false teachings and deception.
After the time of trouble and deception the Lord Himself will return. This will be unmistakable, you will not need a degree in theology to discern it, you will not need any special knowledge or wisdom to see it. Christ will return with great power and glory. And He will send His angels to gather up the elect. Now see all the warnings and instructions given already, and that will be given. Being part of the elect is in no way a ground for complacency. It is an encouragement to us that whatever opposition or trials we may face, we can get through them and Christ will come and rescue us.
Verse 30 is another problematic one. I take it as follows. The events of AD66-70 would happen, showing the truth of all this talk, and this is "proof" that Jesus words regarding the final end times and His return are equally reliable. His words will never pass away.