Learn Polish: Ona ma mało pieniędzy.

It changes because of the word mało (little/few). In Polish, basic quantifiers like dużo (a lot), mało (little), and trochę (a little bit) require the noun that follows them to be in the Genitive case. Pieniędzy is the Genitive form of the word for money.

Is pieniędzy a plural form? In English, "money" is singular.

Yes, it is plural! Unlike in English, the Polish word for money (pieniądze) is always plural. You can think of it as saying "coins" or "funds". Because of this, after mało, we specifically use the Genitive plural form, which is pieniędzy.

Could I use the word mała instead of mało, since ona (she) is feminine?

No, because mało is a fixed word for quantity meaning "little" or "not much". It never changes to match the gender of the subject. Words like mały or mała are adjectives used for physical size (meaning "small"), so using them here would sound like "she has physically small money".

I learned that the verb "to have" is mieć. Why is it ma in this sentence?