Learn Swedish: Den nya har en lång garanti, men den gamla hade ingen. - The new one has a long warranty, but the old one had none.

English uses 'one' as a placeholder so the adjective does not stand alone, but Swedish does not need this. You simply use the freestanding definite article (den or det) followed by the definite form of the adjective (nya or gamla). The noun is left out completely when it is already clear from context.

How do we know what gender the missing objects are?

The word den gives it away! Because the sentence uses den nya and den gamla, we know the speaker is referring back to an en-word, like en maskin (a machine) or en telefon (a phone). If they were talking about an ett-word, like ett kylskåp (a fridge), it would be det nya and det gamla.

Why do the adjectives nya and gamla end in an 'a'?

Whenever you use a definite article like den or det, the adjective must take its definite form. For most adjectives, including ny and gammal, the definite form ends in -a. Note how gammal drops its second 'm' and its 'a' to become gamla.

Why does the sentence end with ingen instead of inte?