"You would have served them to her."
Translation:Vous les lui auriez servies.
Yes, that's correct.
When not written in the imperative, the order is always:
me/te/nous/vous (before) le/la/les/l' (before) lui/leur
You can find more information here:
It is incorrect as the past participle should still agree with the direct object "les". Without context we don't know if "them" is feminine or masculine but we do know that it is plural.
So both "Tu les lui aurais servis" and "Tu les lui aurais servies" should be accepted here
It doesn't help though, that if you make a mistake with "tu", Duo doesn't show you what you got wrong but pushes for the "vous" version instead. I typo'd "tu...aurait", had to figure out my mistake on my own (as Duo just pushed for "vous... auriez"), which I could do here, but doubtlessly many learners would not always realize what their exact mistake was.
Yes. Most French verbs accept an indirect object pronoun, such us servir but there are a few that do not (such as penser à) and those verbs will accept a disjunctive pronoun.
For an example of the verbs that do not accept an indirect object, see the bottom of this page:
It rejected "tu les aurais servis à elle". It "corrected" it to "Vous les lui auriez servies." First, there is no reason to change "tu" to "vous". Second, there is no reason to assume that the gender of "them" is feminine. Third, I used "à elle" instead of "lui" to capture the gender of the recipient. Reported 1 August 2018.
We have to make the agreement in gender and number between the direct object les and the past participle of servir:
- servi - singular masculine
- servie - singular feminine
- servis - plural masculine
- servies - plural feminine
les is the plural of both masculine le and the feminine la, therefore it can be either servis or servies.
Also, tu (singular you) has no accent in French.