Does the Conjunctive Phrase 'Une Fois Que' Need the Subjunctive?

Une fois que ("once that") is a type of conjunctive phrase that does not take the subjunctive because it expresses facts, which are considered certain; this makes the subjunctive unnecessary in this case. On the other hand, there are many more conjunctive phrases that do express uncertainty and, thus, do require the subjunctive.

The subjunctive mood is dedicated to expressing actions or ideas that are uncertain and subjective, such as will/wanting, emotion, doubt, possibility, necessity, judgment. But une fois que and other conjunctive phrases like it are usually followed by the future tense, not the subjunctive, as illustrated below (in English, we would normally use the present):  

   Je l'achèterai une fois qu'il arrivera.
   I'll buy it once he arrives.

   Mangeons quand il arrivera.
   Let's eat when he arrives.

   Une fois que tu aura compris, tout sera plus facile.
   Once you've understood, everything will seem easier.

Note that there are a number of idiomatic expressions that contain the word "fois," such as une fois, cette fois-ce, bien des fois and encore une fois. But they are not relevant to understanding the verb tenses that the conjunction "un fois que" requires.

Here are more conjunctions that do not take the subjunctive because they express facts, which are considered certain and, thus, contrary to the subjectivity of the subjunctive:

• ainsi que > just as, so as
• alors que > while, whereas
• après que** > after, when
• aussitôt que** > as soon as
• car > since, because
• en même temps que > at the same time that
• depuis que > since
• dès que** > as soon as, immediately
• lorsque** > when
• parce que > because
• pendant que > while
• plutôt que > instead of, rather than
• puisque > since, as
• quand** > when
• tandis que > while, whereas
• une fois que** > once

**These conjunctions would be followed by the future tense in French, while in English we use the present tense.