Learn Hardcore Spanish (Latin America): Mi hermano construyó un pequeño robot y lo controla con su teléfono. - My brother built a small robot and controls it with his phone.

Because construyó is the third-person singular preterite form of construir. In Spanish preterite tense, verbs whose stem ends in a vowel add a y in the third-person forms (él/ella), and they carry an accent on the final ó to show the stress falls there and to distinguish it from the present-tense construyo (I build).

Why do we use the preterite tense (construyó) instead of the present or another past tense?

The preterite is used because the action of building the robot is viewed as a completed, one-time event. If we wanted to emphasize the ongoing nature of building, we might use the imperfect (e.g., construía) or the present perfect (e.g., ha construido), but here the speaker simply states a finished action in the past.

Why is the adjective pequeño placed before the noun robot instead of after it?

In Spanish, most adjectives come after the noun, but certain common adjectives—especially those of size, quantity, or inherent qualities—often precede the noun for style or emphasis.

Un pequeño robot (emphasis on “tiny robot” as a whole concept)
Un robot pequeño (more neutral, describing its size)

Both orders are correct; placing pequeño before is slightly more idiomatic when casually introducing a small gadget.

What is the role of the lo in lo controla?