Learn Danish: Han ville ikke have de plettede bukser på. - He did not want to wear the stained trousers.

The base form of the adjective is plettet (stained or spotted). Because it modifies a plural noun (bukser) and follows the definite article de, it must be in its plural and definite form. For adjectives ending in -et, you form this by changing the ending to -ede.

Why is på placed at the very end of the sentence?

The phrase for "to wear" is at have på (literally "to have on"). In Danish, this is a separable verb phrase. When you specify the item of clothing, the object (de plettede bukser) must go between have and på.

Does the word bukser always require a plural adjective?

Yes. Just like the English word "trousers" or "pants", bukser is grammatically plural. That is why the sentence uses the plural article de and forces the adjective into its plural form (plettede).

Could I use gå med instead of have på to mean "wear" here?

You can use at gå med for wearing something as a general habit, but at have på is preferred for what someone is wearing at a specific moment. Here, ville ikke have ... på means he refused to put them on for this specific occasion.