Learn Italian: Quegli enigmi sono i peggiori. - Those enigmas are the worst.

Enigma is a masculine noun of Greek origin. Even though it ends in -a in the singular (which usually signals a feminine noun), these specific Greek loanwords ending in -ma are masculine. Because it is masculine, the plural changes the ending from -a to -i, giving us gli enigmi.

Why do we use quegli for 'those' instead of quei?

The forms of the demonstrative adjective quello ('that'/'those') change to exactly match the definite article the noun would take. Because enigmi is masculine plural and starts with a vowel, its 'the' word is gli (gli enigmi). Therefore, 'those' becomes quegli to match.

How do we know i peggiori means 'the worst' rather than 'the worse'?

In Italian, you form relative superlatives ('the best', 'the worst', 'the tallest') by simply adding a definite article (il, la, i, le) before the comparative word. Peggiore means 'worse', but adding the article makes it 'the worst'. Since enigmi is plural, we use i peggiori.

If enigmi is masculine, why doesn't the adjective end in -o to match it?