Matthew 24:34 is Jesus talking about the "Coming of the Son of Man" or His Return to Earth. Verse 34 is talking about the generation of the latter days and before His return.
Mark 9:1 is talking about John. If you knew Revelation you would know that John saw a vision of Christ returning in Great Power. And John saw this on the Isle of Patmos as an old man and then he wrote Revelation.
Matthew 10:23 Go back to verse 15 where Jesus speaks "in the day of judgment" so Jesus is generalizing to His disciples and of course there are more than 12 because we are also His disciples today and Jesus is speaking of His Second Coming or Jesus is speaking of Him coming back after His resurrection and He does this also. He either speaks of the Second Coming or coming back after His death, burial and resurrection. Verse 32 and 33 speak of "whosoever" which means more followers than just the 12 Disciples.
Jesus did come back from the dead in the lifetime of His disciples, because after this statement was Christ's death, burial and resurrection.
And Mark 14:61-62 is confirmed in Revelation 1:7 "every eye shall see Him, even those who pierced Him."
Revelation 1:7 "Behold, He cometh with clouds; and every eye shall see Him, and they also which pierced Him; and all kindreds of the earth shall wail because of Him, Even so, Amen.
Jesus was telling us the future. We cannot read Scripture without the Holy Spirit who teaches us all things concerning Christ Jesus. Hopefully, your mom gave you a King James Bible and my friend, there is more than enough evidence to prove that Jesus Christ walked this earth and went to the Cross of Calvary for you and me. Scripture tells us "Believe on the Lord Jesus Christ and thou shalt be saved." Jesus is the Savior sent down from God as God's only begotten Son to win back eternal life for you and me. Adam, Eve and Satan lost eternal life for us in the Garden of Eden, but Jesus came to win it back and that all through Him might be saved. There is no other name under heaven in which Salvation comes. The Holy Bible is a magnificent Book. It says Moses wrote the first 5 books, but the truth is God wrote the first 5 books and all the others. Moses was not with God in the beginning, so God had to tell Moses what to write. The prophets of God have never been wrong. They prophesy the future, because God gave it to them to write.
God either told them what to write or God gave them a vision and told them to write what they saw. I've studied the Bible for years and it will come alive for you. Do you know that until the 19th Century the Bible was understood and accepted by all professed Christians as the Word of God? The theory of human evolution has been a frontal attack by the Bible's critics to the point of the Chart showing apes progressing into a man. The truth is God did not breathe His spirit into apes, but God did for man and He made man in His image. He said man is to rule over the apes, the fish, the fowl the animals on the earth and they are inferior to man in God's sight. God created these creatures and beasts for man's pleasure, but in Revelation 4:11 KJV God created man for His pleasure. And I am very thankful God is Holy and that He loves us. God's plan of redemption was given to Adam, Eve and Satan in Genesis 3:15 where it says Jesus will crush Satan's head and Satan will bruise Jesus' heel. That's what happened on the Cross.
It would be helpful for you to read "The Chemistry of the Blood" by Martin R. DeHaan, M.D. Free and online are 5 sermons on "The Life Is In The Blood." My Lord God is amazing. Dr. DeHaan has since passed on in 1965, but this is an amazing read about "blood." The Bible is a bloody book and the man of God explains it. May the Lord make Himself known to you.