Я забыл сдачу у кассы, потому что спешил.

I forgot the change at the cash desk because I was in a hurry.

Breakdown of Я забыл сдачу у кассы, потому что спешил.

• я: I
• потому что: because
• у: at
• забыть: to forget

Questions & Answers about Я забыл сдачу у кассы, потому что спешил.

Why is забыл used here, not забывал?

Забыл is the perfective past tense: it describes a single completed event—you ended up leaving the change behind.
Забывал is imperfective and usually means a repeated/habitual action (I used to forget...) or focuses on the process (I was forgetting...), which doesn’t fit this one-time situation.

What exactly does сдача mean here? I’ve seen it mean “handing in” too.

Сдача has multiple meanings depending on context. Here it means (cash) change—the money you get back after paying.
It can also mean things like handing in/ сдача экзамена (taking/sitting an exam), but with a shop/cash-register context, сдача = change is the natural interpretation.

Why is сдачу in the accusative case?

Because забыть (to forget/leave behind) takes a direct object in the accusative:
забыл (что?) сдачу = forgot (what?) the change.
Also, сдача is inanimate, so its accusative singular form matches the nominative in shape, but here you can still tell it’s accusative from the verb pattern.

What does у кассы mean literally, and why is in genitive?