Learn Hardcore Arabic (Modern Standard): اشترت أمي هدية مختلفة من المتجر الذي في هذا الشارع. - My mother bought a different gift from the shop that is on this street.

This is a very normal word order in Modern Standard Arabic. Arabic often uses verb + subject + object order, so اشترت أمي means my mother bought.

You can also say أمي اشترت..., but that puts a little more focus on my mother. The version in your sentence sounds very natural for neutral narration.

How does اشترت mean she bought?

The basic past-tense verb is اشترى = he bought. When the subject is feminine singular, Arabic adds ـتْ in the past tense, giving اشترت = she bought.

So here:

اشترى = he bought
اشترت = she bought

Because أمي is feminine, the verb must be feminine too.

Why is the verb feminine even though the subject comes after it?

Because the subject is أمي, and mother is feminine. In Arabic, the past-tense verb still shows gender, even when the subject comes after the verb.

So اشترت أمي is literally bought-she my mother, but in natural English that becomes my mother bought.

How does أمي mean my mother?