"Hon tycker om klänningen."

Translation: She likes the dress.

[Stefanos714672]

And "the dresses"? How is it?

[Robbadob]

En klänning = a dress (en-word)
Klänningen = the dress
Klänningar = dresses
Klänningarna = the dresses

Sadly, these are not interchangeable.
https://www.duolingo.com/skill/sv/Definite-forms/tips-and-notes
https://www.duolingo.com/skill/sv/Plurals/tips-and-notes

[VincenzMss]

Tycker om is wrong. Many swedes here told me that they use gillar

[sotnosen93]

Both are acceptable. They're pretty much interchangeable except "gillar" might be a teensy bit more casual, which is probably why a lot of Swedes told you that's what they use.

[Belyynn]

How is klänningen pronounced?

[Belyynn]

Is it klan-ning-n