Learn Swedish: Min gamla dator är såld.

In this sentence, the past participle såld is being used as a predicative adjective describing the computer. Because dator is a singular en-word, the participle takes the matching en-word form såld. If we were describing an ett-word like ett hus (a house), it would be sålt, and for plural nouns it would be sålda.

Wait, isn't the past tense of "to sell" sålde or sålt? Why are we using såld?

Sålde is the simple past tense for an action (e.g., Jag sålde den = I sold it). Sålt is the supine form used after a "have" verb (e.g., Jag har sålt den = I have sold it). But here, "sold" describes the current state of the computer. When a verb acts as a descriptive adjective after är (is), we use the past participle, which is såld for en-words.

Why does the adjective have an "-a" at the end (gamla) if we are only talking about one computer?

Whenever you use a possessive word like min (my), the adjective that follows is forced into its definite form. For most adjectives, the definite form adds an -a to the end. So even though it is just a single computer, gammal changes to gamla.

Since we used the definite adjective form (gamla), why isn't the noun also definite ()?