Learn Spanish (Latin America): Como necesitaba una aguja, habrá sacado este alfiler del maniquí. - Since he needed a needle, he must have taken this pin out of the mannequin.

This is the "future perfect of conjecture," which is the focus of your current lesson! In Spanish, you can use the future perfect (habrá + past participle) to make a strong guess or assumption about an action completed in the recent past. While it literally means "will have taken," native English speakers express this same idea by saying "must have taken" or "probably took."

Could I use Porque instead of Como to start this sentence?

Generally, no. When you want to place "because" or "since" at the very beginning of a sentence to introduce a reason, Spanish requires you to use Como. You would only use porque if you flipped the sentence order: Habrá sacado este alfiler porque necesitaba una aguja.

Why use the imperfect tense necesitaba instead of the preterite necesitó?

The imperfect necesitaba is used here to describe an ongoing background condition. The person's "needing a needle" wasn't a sudden, completed event with a clear start and end; rather, it was the ongoing state of mind or circumstance that set the stage for him taking the pin.

What is the difference between an aguja and an alfiler?

An aguja is a needle, typically used for sewing and featuring an eye for thread. An alfiler is a pin, which has a solid head and is used to temporarily hold pieces of fabric together. This explains the context of the sentence: lacking a needle, the person grabbed a pin off the dressmaker's mannequin.