"She likes blue dresses."
Translation:Le piacciono i vestiti azzurri.
The verb "piacere" in Italian works differently from English.
She likes something -> A lei (= Le) piace qualcosa. Something/qualcosa is the subject.
A me piacciono i libri = Mi piacciono i libri (libri singular)
A te piace viaggiare = Ti piace viaggiare (viaggiare is a verb used as a noun, singular)
And so on. Please ask me if you have further doubts.
It's because the dresses are the subject here and also plural. "The dresses please her". You have the same constructions in French (plaire) and German (gefallen) - it takes some getting used if your mother tongue is English but it will become second nature after a short while.
I was about to report the “piaciono” in the conjugation table as an error until I checked elsewhere and found it listed as an alternative, but I can’t seem to find a more complete explanation anywhere. (This was after I was marked wrong for answering “A lei piaciono i vestiti azzurri.” and then correct for changing it to “A lei piacciono i vestiti azzurri.”.) Are they supposed to be interchangable, or are there times when one is correct and the other isn’t?