Learn Spanish (Latin America): El otro día vi a tu hermano.

This is the "personal a." In Spanish, whenever the direct object of a verb is a specific person (like a brother), you must put an a right before it. It doesn't translate to anything in English, but leaving it out sounds incorrect to native speakers.

Since this lesson is about the past perfect, why is vi in the simple past (preterite) instead of the past perfect (había visto)?

The past perfect is used to describe an action that happened before another past action. This sentence only describes a single past event that happened at a specific time (el otro día). Without a second, more recent past action to compare it to, you just use the simple preterite (vi).

Why doesn't vi have an accent mark on the "i" like comí or escribí?

The verb ver is slightly irregular. Normally, the "yo" form of regular ER and IR verbs gets an accent mark in the preterite tense. However, because vi is only one syllable, the stress is already obvious. Spanish spelling rules state that one-syllable words don't get accent marks unless they are needed to tell two identical words apart.

Does the phrase El otro día usually trigger the preterite tense?