What is the structure of this sentence?
First come, first served
"come" and "served" seem to be v3 but "first" means someone. Therefore I couldn't understand why "first come" instead of "first came"
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That sentence employs two literary or rhetorical techniques with the aim of creating a stronger, more memorable statement.
The first technique is called anaphora, which is the repetition of a word or phrase at the beginning of successive clauses.
Another example of anaphora (from Abraham Lincoln's 2nd Inaugural Address):
With malice toward none; with charity for all; with firmness in the right,...
The "First ... first" anaphora makes the statement stronger by repetition and therefore easier to remember.
The second technique is called ellipsis, which is the omission of words from a sentence's syntax which would otherwise be readily understood. It is used here to shorten the sentence and thereby make it a stronger statement, packing the meaning into fewer words.
[Full] The flood destroyed my house. Why did it destroy mine?
[Elliptical] The flood destroyed my house. Why mine?
The "First come, first served" sentence without ellipsis would probably be something like the following, though there are other possibilities (elided words shown in brackets):
[Those who are] first [to] come [shall be the] first [who are/will be] served.
Notice how much pithier, how much more memorable, how much stronger is First come, first served than the complete sentence would be. Total words: four. Total impact: much, much greater.