En 1969, on est part / parties a la conquete de la Lune

Question

Why is the subjunctive given in this same test question answer as "parties" in one kwiz and "parti" in another on subsequent days?!  On est obviously is "we", but there is no indication of genders(s) of "We" so why was this parties yesterday and parti today?!

Accepted Answer

Hi Diane, 
there 's no subjunctive in sight in this sentence. The verb is est, which is the auxiliary verb to form the perfect tense with the participle parti(s).
On est parti à la conquête de la Lune. -- We (i.e., humanity) set out to conquer the moon.
The participle is in singular (i.e., parti, and not partis) because the "we" is a general we, denoting humanity and not a particular group.