"I do not think she prepared this phase."
Translation:Je ne pense pas qu'elle ait préparé cette étape.
Doesn't "a préparé" = "has prepared" whilst "ait préparé" = "had prepared"?
This is in the past subjunctive (instead of the past indicative) as it has been triggered by penser que in the negative.
Had it been a positive statement -
"I think she has prepared this phase" →
Je pense qu'elle a préparé cette étape
- the indicative would have been fine but, as it is negative, the subjunctive is required:
"I do not think she prepared this phase" →
Je ne pense pas qu'elle ait préparé cette étape.
When you are unsure which to use, this tool is invaluable:
"Penser que" used in the negative requires it to be followed by the subjunctive. The imperfect subjunctive is a literary verb form, meaning that it's reserved for formal, written French – mainly literature, but also history and journalism and is not taught on Duolingo.
I got the gender of étape wrong...thought it was masculine. So the Duo program told me that the right answer was plateau. Looking up plateau, nothing like phase is in the dictionary so I am sure that's wrong.
Wow, that is some round about thinking by Duo. A synonym of phase is stage and another word for a (theatrical) stage in French is plateau which is, indeed, masculine. See these:
Ghost in the machine?