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Matthew 16:28 Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
Here is an excerpt from an online Bible Commentary ["The Believer's Bible Commentary] which answers your question perfectly:
16:28 He next made the startling statement that there were some standing there with Him who would not taste death before they saw Him coming in His kingdom. The problem, of course, is that those disciples have all died, yet Christ has not come in power and glory to set up His kingdom. The problem is solved if we disregard the chapter break and consider the first eight verses of the next chapter as an explanation of His enigmatic statement. These verses describe the incident on the Mount of Transfiguration. There Peter, James, and John saw Christ transfigured. They were actually privileged to have a preview of Christ in the glory of His kingdom.
We are justified in viewing Christ's transfiguration as a pre-picture of His coming kingdom. Peter describes the event as "the power and coming of our Lord Jesus Christ” (2Pe_1:16). The power and coming of the Lord Jesus refer to His Second Advent. And John speaks of the Mount experience as the time when " ... we beheld His glory, the glory as of the only begotten of the Father” (Joh_1:14). Christ's First Coming was in humiliation; it is His Second Coming that will be in glory. Thus, the prediction of verse 28 was fulfilled on the Mount; Peter, James, and John saw the Son of Man, no longer as the humble Nazarene, but as the glorified King.
I hope you have found this helpful.