Я оставил мелочь на столе, чтобы не забыть её.

I left the small change on the table so that I wouldn’t forget it.

Breakdown of Я оставил мелочь на столе, чтобы не забыть её.

• я: I
• стол: the table
• на: on
• не: not

Questions & Answers about Я оставил мелочь на столе, чтобы не забыть её.

Why is оставил used here, not оставлял?

Оставил is perfective aspect (completed, one-time action): I left it (and the leaving is viewed as done).
Оставлял is imperfective aspect and would usually mean a repeated habit or process (I used to leave… / I was leaving…) or background description. In this sentence you’re talking about one specific completed action, so оставил fits best.

What case is мелочь in, and why doesn’t it look different?

It’s the direct object of оставил, so it’s in the accusative case.
For many feminine nouns ending in a soft sign (-ь), nominative and accusative look the same: мелочь (Nom) = мелочь (Acc).

What does мелочь mean in real usage? Is it just “small things”?

Мелочь commonly means:

• small change / coins (very frequent in everyday speech)
• small items / trifles / little things (more general)

In context with оставил … на столе, it very often implies some coins / small change left on the table.

Why is it на столе and not ?