"Copiii au displăcut-o pe profesoara lor."
Translation:The children disliked their teacher.
In the accusative, when being a direct object and a substitute for nouns that designate human beings, the unstressed (clitic) forms of the personal pronoun (mă - te - îl - o - ne - vă - îi - le) can be used, alone or accompanied by the equivalent stressed (full) form (pe mine - pe tine - pe el - pe ea - pe noi - pe voi - pe ei - pe ele).
- I love him = (Eu îl iubesc pe el) or (Eu îl iubesc)
- I love her = (Eu o iubesc pe ea) or (Eu o iubesc)
But the sentence that we have here does not have sense at all without pe profesoara lor:
- Copiii au displăcut-o / The children disliked her (who they disliked?)
- Copiii au displăcut-o pe profesoara lor / The children disliked their teacher.
So that o is like her in english, and pe profesoara lor is stressed part of the sentence. Literally the sentence means: "The children disliked her their teacher" - it does not make sense in english.
In the compound perfect of the indicative, the unstressed forms precede the auxiliary. When combining the unstressed form of the pronoun and the auxiliary a avea that forms the coumpound perfect, the elison and hyphenation are required: mă + a = m-a vazut; îl + a = l-a vazut, for example.
However, the unstressed form of the 3rd person singular feminine o comes after the verb (past participle): a vazut-o, for example.
- Copiii au displăcut-o pe profesoara lor. (the teacher is a woman)
- Copiii l-au displăcut pe profesorul lor. (the teacher is a man)
Didn't realize that plăcea/displăcea could even be used with a direct object. Wouldn't it be more common to express this with the dative case? That said, I'm not sure what that would event look like. Something like 'Profesoara le-a displăcut copiilor.' or 'Profesoara copiilor le-a displăcut.'?
A Romanian speaker would understand what you mean by using "a displăcea" as a transitive verb, but this is not standard nor common usage. I think Duolingo is pushing the boundaries of the language here. (Or is just plain wrong.)
The correct way to say the above sentence is either of the following:
- Copiii nu au plăcut-o pe profesoara lor. = The children did not like their teacher.
- Copiilor le-a displăcut profesoara lor. = The children disliked their teacher.
PS: Your versions are also okay, but they could mean slightly different things depending on context and emphasis. For now, you should stick to the ones I wrote above.