Learn Italian: Due settimane fa, abbiamo scavato nella montagna.

The word fa translates to "ago", and it works exactly the same way as in English! It is always placed immediately after the amount of time. So, due settimane fa matches the word order of "two weeks ago" perfectly.

Could I put due settimane fa at the end of the sentence instead?

Yes, you can! Just like in English, time expressions can often go at the beginning or the end of a sentence. Abbiamo scavato nella montagna due settimane fa is also completely correct and natural.

Why do we use the passato prossimo (abbiamo scavato) here instead of the imperfect tense?

The phrase due settimane fa tells us this was a specific, completed event in the past. When you pinpoint an action to a set timeframe like this, Italian uses the passato prossimo. You would use the imperfect tense if you were describing a continuous past habit, like "we used to dig."

Why is it nella montagna and not just in montagna?

Nella is a combination of in (in) + la (the). Because you are talking about digging inside a specific mountain here, you need "in the", which merges into nella. The phrase in montagna without the article is a set expression that generally means "to the mountains" or "in the mountains" (like going on vacation).