Learn German: Ich brauche einen Ersatz mit einer langen Garantie. - I need a replacement with a long warranty.

This is because of the preposition mit, which always forces the dative case. In the dative case, whenever an adjective comes after an article like einer (or einem, meinem, keinem), the adjective always takes an -en ending.

Why is it einen Ersatz (accusative) but then einer langen Garantie (dative)?

The verb brauchen (to need) requires a direct object in the accusative case, so the masculine noun Ersatz becomes einen Ersatz. But the second part of the sentence is controlled by the preposition mit, which always takes the dative case, completely independent of the verb.

How can I tell the gender of Garantie just by looking at this sentence?

You can spot it from the article einer. Since we know mit requires the dative case, einer is the dative form for feminine nouns. As a helpful rule of thumb, almost all German words ending in -ie (like die Familie or die Batterie) are feminine.

Can Ersatz be used for replacing people, or just objects?

It works for both! Der Ersatz is a very versatile word that means replacement, substitute, or spare. You can use it for a broken appliance, a missing recipe ingredient, or a coworker filling in for you on a shift.