Learn Hardcore Portuguese (Brazil): A lista foi feita pela minha mãe, então foi fácil comprar tudo. - The list was made by my mother, so it was easy to buy everything.

foi = past of ser (to be), used to form the passive
feita = past participle of fazer (feito/feita)
So A lista foi feita... = The list was made... (completed action in the past).

Why is it feita and not feito?

Because the participle agrees with the noun it describes. Lista is feminine singular, so the participle must be feita.

o relatório foi feito (masc. singular)
a lista foi feita (fem. singular)
as listas foram feitas (fem. plural)

Why is pela minha mãe used instead of por minha mãe?

Pela is por + a (a contraction). Since mãe is preceded by the definite article a (here included inside minha in the sense of a minha mãe), Portuguese contracts it:

por + a = pela
Similarly: por + o = pelo, por + os = pelos, por + as = pelas.
In passive sentences, por/pelo/pela commonly introduces the “agent” (who did it).

Can I also say A lista foi feita por minha mãe? Is it wrong?

It’s not necessarily “wrong,” but it changes the feel.

pela minha mãe sounds more natural/typical because Portuguese often uses the article with family members in this kind of structure.
por minha mãe can sound a bit more formal, emphatic, or like you’re deliberately omitting the article.

Why is minha mãe used instead of a minha mãe?

Both exist. In Brazil, you’ll often hear minha mãe without the article in everyday speech. But when it follows por, the contracted form pela minha mãe is extremely common and natural (because pela already contains a).

What does então mean here, and where can it go in the sentence?

Here então means so / therefore (showing a result).
It often appears in the middle like this:

..., então foi fácil...
You can also place it at the start of the second clause for emphasis:
..., então, foi fácil comprar tudo.
Or:
A lista foi feita pela minha mãe. Então foi fácil comprar tudo.

Why does the sentence repeat foi twice?

Because there are two separate past statements:
1) A lista foi feita... (event: the list got made)
2) foi fácil comprar tudo (evaluation: it was easy)
Portuguese doesn’t avoid repetition of foi the way English sometimes might; it’s normal.

In foi fácil comprar tudo, what is the subject? Why isn’t it foram fáceis?

The structure é/foi + adjective + infinitive is impersonal:

Foi fácil comprar tudo. = It was easy to buy everything.
There isn’t a normal noun subject, so the verb stays singular (foi). You don’t make it plural because nothing plural is “the subject” grammatically; comprar tudo is an infinitive clause functioning like an idea.

Could I say Foi fácil comprar todos instead of tudo?

Not in this meaning. tudo means everything (a pronoun referring to “all the things/items”).
todos usually means everyone (people) or all (plural) but it normally needs a noun or a clear plural reference:

comprar tudo = buy everything (all items)
comprar todos os itens = buy all the items
comprar todos would sound like “buy them all,” and without context it’s odd (and could even suggest people).

Why is it comprar and not para comprar?

Portuguese often uses easy/difficult + infinitive without para:

É difícil entender isso.
Foi fácil comprar tudo.
You can use de in some cases (especially with certain adjectives), but fácil + infinitive without a preposition is very common and natural.

Is there any difference between A lista foi feita pela minha mãe and Minha mãe fez a lista?

Yes: passive vs active focus.

Minha mãe fez a lista (active) focuses on who did it.
A lista foi feita pela minha mãe (passive) focuses on the list (the thing affected), and the doer is added with pela minha mãe.
Both are correct; the choice depends on what you want to emphasize.

What’s the difference between foi feita (with ser) and estava feita (with estar)?

foi feita (ser + participle) describes the action/event: the list was made (someone made it).
estava feita (estar + participle) describes the resulting state: the list was already done/ready.
Example: Quando cheguei, a lista já estava feita. = When I arrived, the list was already done.

Does minha mãe need a capital letter (like Mãe)?

Normally mãe is lowercase: minha mãe.
You might capitalize Mãe in direct address or as a title in some contexts, but in a normal sentence describing your mother, lowercase is standard.

Can pela minha mãe mean something other than “by my mother”?

Yes, por/pelo/pela can also mean things like through, around, for, because of, depending on context. But in a passive sentence (foi feita), pela minha mãe is straightforwardly the agent: by my mother.

Why is there a comma before então?

Because the sentence has two independent clauses:

A lista foi feita pela minha mãe
foi fácil comprar tudo
The comma helps mark the pause and shows that então is linking the cause and result. It’s good style, especially in writing.