Adversarius ne semel quidem vincit.

Adversarius means opponent, enemy, or adversary.

It is the subject because it is in the nominative singular form. The ending -us is a very common nominative singular ending for a second-declension masculine noun.

So here:

• adversarius = the opponent / an opponent
• it is the one performing the action of vincit

Latin often leaves out articles entirely. It has no definite article (the) and no indefinite article (a/an) as separate words.

So adversarius can mean:

• the opponent
• an opponent
• sometimes just opponent, depending on context

English has to choose one, but Latin does not.

Vincit is from the verb vinco, vincere, meaning to conquer, defeat, or win.

Here vincit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

So it means:

• he/she/it wins
• he/she/it is winning
• sometimes in natural English, he/she/it does win

Because the subject is adversarius, the sense is the opponent wins.

English often needs the helping verb do for negatives: does not win.

Latin does not need that kind of helper verb. The verb vincit already contains the person and number:

• vin- = the verb stem
• -cit = he/she/it in the present tense

So Latin can simply negate the statement without adding a separate word meaning does.

Semel means once or one time.

By itself:

• semel vincit = he wins once

In this sentence it is part of the expression ne semel quidem, which means not even once.

Ne ... quidem is a common Latin expression meaning not even.

It usually works like a pair that surrounds the word being emphasized.

Here:

• ne semel quidem = not even once

So the sentence says that the opponent does not win even once.

Because ne ... quidem is the normal idiom for not even in Latin.

Compare the ideas:

• non = ordinary not
• ne ... quidem = not even

So:

• non vincit = he does not win
• ne semel quidem vincit = he does not win even once

This is more emphatic than a simple negation.

Also, in this expression, ne is not acting alone in the usual sense of lest or a negative command. It is part of the set phrase ne ... quidem.

That is how the idiom normally works. The emphasized word goes between them.

So Latin often does this:

• ne X quidem = not even X

In your sentence, X is semel:

• ne semel quidem = not even once

If a different word were emphasized, that word could be placed between ne and quidem instead.

Yes, Latin word order is much freer than English word order.

The sentence is:

• Adversarius ne semel quidem vincit.

A very literal order would be:

• The opponent not-even once wins.

This order is natural in Latin because:

• the subject comes first: adversarius
• the emphatic phrase ne semel quidem comes before the verb
• the verb often comes at or near the end

Latin could rearrange the words for emphasis, but the meanings of the forms would still show who does what.

It is singular.

The ending -us here shows nominative singular. If it were plural, you would expect adversarii.

So:

• adversarius = the opponent
• adversarii = the opponents

That is why vincit is also singular: he/the opponent wins.

It can be very close in meaning, but literally it means the opponent does not win even once.

That is slightly more specific than a plain never wins. The Latin emphasizes the idea of not even one single time.

So depending on context, English might render it as:

• The opponent does not win even once
• The opponent never wins

But not even once is the most literal reflection of ne semel quidem.