Questions & Answers about Hans syster tycker om den ljusa månen.
Why are both 'den' and the '-en' ending on 'månen' used just to say 'the bright moon'?
This is a classic example of Swedish double definiteness. When a noun has an adjective in front of it, Swedish marks 'the' twice: once with the freestanding article den before the adjective, and again with the definite ending -n or -en attached to the noun itself (månen).
Why does 'ljus' have an '-a' at the end here (ljusa)?
Whenever you use the definite form (talking about 'the' specific thing rather than 'a' thing), the adjective must also take its definite form. For almost all adjectives, this simply means adding an -a to the end. So, ljus becomes ljusa to match den and månen.
Why does the sentence use tycker om instead of just tycker?
By itself, the verb tycker means 'to think' in the sense of having an opinion (like 'I think the moon is pretty'). To mean 'to like', you must pair it with the particle om. Together, tycker om is a set phrase meaning 'likes'.
Since we are talking about his specific sister, why isn't it Hans systern?