Read Mal 4 5 to 6
Behold I send you Elijah the prophet before the coming of the great and dreadful day....etc...
Before the trib...John did not fulfill all of it...so another man in the spirit and power of Elijah must come...and John did not restore all things..the future Elijah does...you can Google it and study it..there is a fair amount of info on it
That great and dreadful day of the Lord is when Jesus came and was crucified, and salvation came to all who would believe on him.
Read the full context of Malachi which include verse 6
“Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the Lord . He will restore the hearts of the fathers to their children and the hearts of the children to their fathers, so that I will not come and smite the land with a curse.”
Malachi 4:5-6 NASB
John preached repentance and the coming of the Lord and baptized many
Fulfilling this completely.