Learn Portuguese (Portugal): Assim que tivermos construído a base na superfície, moraremos lá. - As soon as we have built the base on the surface, we will live there.

tivermos construído is the Future Perfect Subjunctive. It is used here after the time trigger assim que (as soon as) to heavily emphasize that the building process must be completely finished before the second action (living there) can happen. While the simple future subjunctive (construirmos) would also be correct, the compound form gives a stronger sense of "once we have completely finished building."

How exactly do you form the Future Perfect Subjunctive used in this sentence?

It requires two parts: the auxiliary (helping) verb ter conjugated in the simple future subjunctive (which is tivermos for the "we" form), followed by the past participle of the main verb (in this case, construído).

Does assim que always trigger a subjunctive tense?

Not always! It triggers the future subjunctive (or future perfect subjunctive) when you are talking about hypothetical or unfulfilled future events, as in this sentence. However, if you are talking about a completed action in the past (e.g., "As soon as we arrived, we slept"), you would use the standard indicative mood because the event is a confirmed fact.

Could I say vamos morar lá instead of moraremos lá in the second half of the sentence?