Mat 17:11 Elias truly shall first come, and restore all things.
Elijah truly starts out and delivers everything.
Christ says this in response to his request not to tell anyone about his appearing to them with Moses and Elijah. However, it also reflects an earlier statement in Mat 11:14 which suggests that John the Baptist was a reincarnation of the prophet Elijah.
Elias is the Greek form of the name of the prophet we call "Elijah." Christ refers to Elijah only here and in Mark as a forerunner or harbinger of the Christ. However, he also appear with Christ along with Moses. More about Christ's use of OT figures in this article.
The word translated as "shall come" primarily means "to start out." It is not in the future tense, but the present. It indicates movement, especially its beginning, without indicating a direction toward or away from anything, so it works either as "come" or "go," but it is more like our phrase "being under way."
The Greek word for "first" appeared in the Greek source used by the KJV translators, but does not appear in the sources we use today, which are much more accurate.
"Restore" is from a Greek, which means "reestablish", "restore", "reinstate," and "return." Its base is a word that means "to stand" or "to set up."
The word translated as "all things" is one word meaning "all", "the whole", "every," and similar ideas. As an adverb, it means "in every way", "on every side," and "altogether."
Christ here refers to the Jewish prediction that Elija would return before the Messiah. The context is Christ's appearance with Moses and Elijah. The statement seems to be a direct statement about a general belief among the Jews in reincarnation, where Elijah was prophesied to come again, which was seen as being reincarnated as someone ese. In answering an earlier statement in Mat 16:14, the apostles describe the people's general belief that Christ himself may have been a reincarnation.
ἔρχεται (verb 3rd sg pres ind mp) "Shall come" is from erchomai, which means "to start," "to set out", "to come", "to go," and any kind of motion. It means both "to go" on a journey and "to arrive" at a place.
καὶ "And" is from kai, which is the conjunction joining phrases and clauses, "and," or "but." After words implying sameness, "as" (the same opinion as you). Used in series, joins positive with negative "Not only...but also." Also used to give emphasis, "even", "also," and "just."