Learn Hardcore Portuguese (Portugal): A torradeira avariou ontem, por isso hoje torro o pão no forno. - The toaster broke down yesterday, so today I toast the bread in the oven.

Yes. Both A torradeira avariou ontem and A torradeira avariou-se ontem are correct in European Portuguese. The reflexive form avariou-se is very common and sounds natural in everyday speech when something stops working “on its own.” The non‑reflexive avariou is also fine and somewhat more neutral/formal. You’ll also hear synonyms:

A torradeira estragou-se ontem. (very common)
A torradeira pifou ontem. (informal/colloquial)

What’s the difference between por isso and porque? And how about então?

por isso = “so/therefore,” introduces a consequence:
A torradeira avariou ontem, por isso hoje torro o pão no forno.
porque = “because,” introduces a cause:
Hoje torro o pão no forno porque a torradeira avariou ontem.
então can also mean “so/then,” but it’s more conversational and can sound less formal than por isso:
A torradeira avariou ontem; então hoje torro o pão no forno.
You’ll also hear por isso é que (“that’s why”) for emphasis:
A torradeira avariou ontem, por isso é que hoje torro o pão no forno.

Do I need the comma before por isso? And should there be a comma after it?

A comma before por isso is standard because it links two independent clauses:
A torradeira avariou ontem, por isso hoje torro o pão no forno.
A comma after por isso is optional and reflects a stylistic pause:
…, por isso, hoje torro o pão… (both with and without the second comma are acceptable).

Why is it torro and not tosto? What’s the difference between torrar and tostar?

Both verbs exist, but torrar is the default for toasting bread in Portugal.

torrar bread is standard: Hoje torro o pão…
tostar also means “to toast/brown,” but it often suggests lighter browning or is used in certain fixed expressions (e.g., tosta mista for a toasted sandwich).
Conjugation note: torrar → eu torro, tu torras, ele/ela torra…

Do I have to say eu torro? Or can I drop eu?

You can drop it. Portuguese is a pro‑drop language, and the verb ending already tells you the subject:

Neutral: Hoje torro o pão no forno.
Add eu for emphasis or contrast: Hoje eu torro o pão no forno (e não tu).

Why is it o pão and not just pão?

Portuguese uses definite articles more than English. With mass nouns like “bread,” the definite article is common when referring to a specific, contextually known item (the bread you have at home): torro o pão.

torro pão (no article) sounds odd.
torro um pão would mean “a loaf/roll,” i.e., one item.
For generic statements: O pão é caro. (“Bread is expensive.”)

What does no forno mean exactly? Could I say ao forno?

no forno = “in the oven” (em + o = no). That’s the natural choice here.
ao forno (from a + o) exists but is mostly found in set phrases or recipe names (more common in Brazil), like frango ao forno. In Portugal, to express location/method, use no forno: torro o pão no forno.

Is torradeira the same as tostadeira?

In Portugal:

torradeira is the standard “toaster” (slot/popup toaster for slices of bread).
tostadeira often refers to a sandwich press/grill (for making tostas, like a panini/press), though usage varies by region/brand.
So for this sentence, torradeira is the correct appliance.

Why use the present tense torro with hoje? Shouldn’t it be future?

Portuguese often uses the simple present with time adverbs to talk about scheduled or intended near‑future actions: Hoje torro o pão… means “Today I’m toasting the bread…”
You can also say Hoje vou torrar o pão… (near future) with a slightly stronger sense of plan/intention.

Could I say Estou a torrar o pão no forno?

Yes, but it changes the meaning.

Estou a torrar o pão no forno = you’re doing it right now (progressive aspect, EP uses estar a + infinitive).
Hoje torro o pão no forno = you plan/expect to do it at some point today, not necessarily at this moment.

Can I move ontem and hoje around? What are natural word orders?

Yes. Adverbs of time are flexible. All of these are natural:

Ontem a torradeira avariou; por isso hoje torro o pão no forno.
A torradeira avariou ontem; por isso hoje torro o pão no forno.
A torradeira avariou ontem; por isso, hoje, torro o pão no forno. (extra commas = extra pauses/emphasis)

Any pronunciation tips (European Portuguese)?

torradeira: [tuʁɐˈðejɾɐ] — the double rr is the strong guttural R [ʁ]; unstressed vowels reduce.
avariou: [ɐvɐˈɾju] or [ɐvɐˈɾjou] — the single r between vowels is a tap [ɾ].
ontem: [ˈõtɐ̃j̃] — nasal vowel at the end.
por isso: [puˈɾisu] — often runs together in speech.
hoje: [ˈoʒ(ɨ)].
torro: [ˈtɔʁu] — double rr = [ʁ].
pão: [pɐ̃w̃] — nasal diphthong.
forno: [ˈfoɾnu] — single r = tap [ɾ].
Natural connected speech will compress vowels and link words: …avariou ontem, por isso hoje torro o pão… flows with few pauses.