Learn Hardcore Portuguese (Brazil): O bolo foi colocado no forno pela minha mãe, e ficou pronto em vinte minutos. - The cake was put in the oven by my mother, and it was ready in twenty minutes.

That’s the passive voice: O bolo foi colocado no forno = “The cake was put in the oven.”
Portuguese often uses the passive when the focus is on the object/result (the cake), not on who did it.
An equally natural active version is: Minha mãe colocou o bolo no forno.

What tense is foi colocado?

It’s pretérito perfeito (simple past) of ser (foi) + a past participle (colocado).
Structure: ser (past) + participle = passive action completed in the past.

Why is it colocado and not colocada?

The past participle agrees with the thing being described in gender/number.
bolo is masculine singular, so: colocado.
If it were a torta, you’d say foi colocada.

What does no forno literally mean, and why is it one word?

no is a contraction of em + o (“in/on/at” + “the”).
So no forno = em o forno → “in the oven.”
Similarly: na = em + a, nos = em + os, nas = em + as.

Why does it say pela minha mãe? Is that “for my mother”?

Here pela marks the agent of the passive (“by”).
pela = por + a.
So pela minha mãe = “by my mother.”
“for my mother” would usually be para minha mãe (or pela minha mãe only in the sense of “on behalf of my mother,” depending on context).

Could I replace pela minha mãe with por minha mãe?

Yes, but it changes the article choice.

pela minha mãe = por + a minha mãe (specific/definite)
por minha mãe (without the article) is also possible, though pela minha mãe is very common and often sounds more natural/explicit.

Why is there an e after the comma? Is that optional?

e means “and,” linking two events:
1) it was put in the oven
2) it got ready in 20 minutes
The comma before e is optional and stylistic. Many writers would omit it:
..., e ficou pronto... or ... e ficou pronto... (both seen).

What does ficou pronto mean exactly, and why use ficar?

ficar often means “to become” or “to end up” in a new state.
So ficou pronto = “(it) became ready / was ready (after that).”
Using ser (foi pronto) would sound wrong here; ser isn’t used to mean “became ready” in this context.

Why is it pronto and not pronta?

Same agreement rule as with colocado: it describes o bolo (masculine singular).
So: pronto. If it were a pizza, it would be ficou pronta.

Does em vinte minutos mean “in 20 minutes” or “within 20 minutes” or “after 20 minutes”?

In this context it usually means it was ready after 20 minutes / in 20 minutes (duration until completion).
Portuguese uses em + time a lot for this “time to finish” meaning.
If you want to stress “it stayed ready for 20 minutes” (duration of a state), you’d more likely use por vinte minutos.

Could I say por vinte minutos here instead?

You can, but it changes the meaning:

ficou pronto em vinte minutos = it took 20 minutes to get ready
ficou pronto por vinte minutos = it stayed ready for 20 minutes (unusual for a cake)

Is there a more natural verb than colocado for putting a cake in the oven?

colocar no forno is correct and common. Depending on style, you might also see:

O bolo foi posto no forno... (more concise; posto is the irregular participle of pôr)
But colocado is perfectly natural in Brazilian Portuguese.