Learn Swedish: Han fixade bilen själv eftersom den var trasig. - He fixed the car himself because it was broken.

The word själv (himself/oneself) acts like an adjective that describes the subject. Since han (he) is just one person, we use the singular form själv. If the subject were plural, like de (they), you would use the plural form själva.

Why do we use den to mean 'it' in the second half of the sentence?

In Swedish, the word for 'it' depends on the noun it is replacing. Here, 'it' refers back to bilen (the car). Because en bil is an en-word, we must use den. If we were talking about an ett-word like ett hus (a house), we would use det instead.

Why is the adjective trasig and not trasigt?

Adjectives have to match the gender of the noun they describe. The word trasig (broken) describes den, which refers to bilen. Since bil is an en-word, the adjective stays in its basic form trasig without getting an extra -t at the end.

Is eftersom the only way to say 'because' here?

You could also use för att or därför att to mean 'because' in this sentence. All of them are common, but eftersom is often used when the reason is a natural explanation for the main action, rather than answering a direct 'why' question.