Learn Spanish (Latin America): Al retroceder, el niño usó el escudo. - Upon backing up, the boy used the shield.

The structure al + infinitive (like al retroceder) is a very common shortcut in Spanish to say "upon doing [something]" or "when doing [something]." It sounds more concise and natural here than using cuando with a conjugated verb, especially because the person doing the action (the boy) is the same in both parts of the sentence.

Who is doing the "backing up" since there is no subject in the first half of the sentence?

The boy (el niño). When you use the al + infinitive structure, the person performing that action is generally the same as the subject of the main sentence. Since el niño used the shield, we automatically know he is also the one backing up.

Why is the main verb usó in the preterite tense?

The preterite is used for completed actions in the past. Here, usó (he used) happened as a specific, sudden action triggered exactly when he backed up. If it were an ongoing, descriptive background action (like "he was using the shield"), we would use the imperfect tense (usaba).

Does the verb retroceder only apply to physical movement?

Mostly yes, retroceder means to go backward, back up, or retreat. You will hear it used for cars reversing or people taking a step back. However, it can also be used metaphorically, such as backing down from an argument or reversing a decision.