Learn Portuguese (Portugal): Tu precisas de perseverança até desenterrares o fóssil milenar. - You need perseverance until you unearth the ancient fossil.

It depends on the word used for 'until'. Here, we use the preposition até, which is followed by an infinitive. We add the -es ending for tu to make it the personal infinitive. If we had used the conjunction phrase até que, then we would have been forced to use the subjunctive (até que desenterres).

Could we just use the regular, uninflected infinitive (até desenterrar) here since the subject is already established as tu in the first half of the sentence?

Yes, technically you can. When the subject of both clauses is exactly the same, using the uninflected impersonal infinitive is grammatically correct. However, European Portuguese speakers frequently choose to use the personal infinitive anyway (desenterrares) because it sounds highly natural and explicitly reinforces who is doing the action.

Can you break down the anatomy of the verb desenterrares?

The root is the noun terra (earth or dirt). By adding the prefix des- (indicating removal or reversal) and the verb suffix -ar, we get the base infinitive desenterrar (to unearth or dig up). Finally, the -es at the end is the personal infinitive suffix for the tu form.

Why use milenar instead of a more common word like antigo for 'ancient'?