Learn Spanish (Spain): Si tenías hambre, ¿por qué no comiste?

You only use the past subjunctive after si for impossible, contrary-to-fact, or highly unlikely conditions (like "If I were rich"). Here, the speaker is talking about a real possibility in the past: you actually might have been hungry. For real past conditions, you just use the regular past indicative tenses.

Why do we use the imperfect for tenías but the preterite for comiste?

The imperfect (tenías) describes the ongoing background state or condition of being hungry. The preterite (comiste) is used for the specific, single action that the person chose not to take (eating) at that particular moment in time.

Could I use the preterite tuviste instead of tenías here?

While grammatically possible, tenías sounds much more natural. Being hungry is typically an ongoing physical state, which naturally triggers the imperfect. Using tuviste would imply a sudden, immediate onset of hunger right at that exact moment, rather than the background state of being hungry.

Why does the inverted question mark (¿) appear in the middle of the sentence instead of at the very beginning?

In Spanish punctuation, when a sentence starts with an introductory clause (like the si condition here), the opening question mark only wraps around the actual question part. It is placed exactly where the question begins, which is right after the comma.