Learn Spanish (Latin America): El juramento será eterno mientras ellos no traicionen el pacto. - The oath will be eternal as long as they do not betray the pact.

In this sentence, mientras sets up a condition that must hold true in the future ("as long as"), rather than stating a current fact. When mientras talks about an anticipated future duration or condition, it triggers the subjunctive (traicionen). If we used the indicative (traicionan), it would mean "while they are not betraying" (a factual statement about what they are doing right now).

I usually translate mientras as "while." Is "as long as" always the best translation when it takes the subjunctive?

Yes, in most cases. When mientras is followed by the subjunctive to express a conditional future duration, "as long as" or "provided that" captures the meaning perfectly. It shows that the first part of the sentence (El juramento será eterno) depends entirely on the second part holding true.

How would the meaning change if the sentence used hasta que (until) instead of mientras?

If you said hasta que traicionen el pacto (until they betray the pact), it implies a strong expectation that they will eventually betray it, and the oath will end then. Using mientras no traicionen (as long as they do not betray) focuses on the ongoing condition for success, leaving open the hope that they never betray it at all.

Do we have to include the subject pronoun ellos here?