Learn Hardcore Portuguese (Brazil): Eu cortei o pão com a faca, mas me cortei também. - I cut the bread with the knife, but I cut myself too.

Cortei is the pretérito perfeito (simple past), used for a completed action: you cut the bread (and you cut yourself) and the action is viewed as finished.
Cortava is the pretérito imperfeito, used more for ongoing/habitual/background past actions (e.g., Eu cortava o pão quando me cortei = I was cutting the bread when I cut myself).

What is the function of o in o pão?

O is the definite article (the). O pão usually implies a specific bread/loaf that’s contextually known (the bread on the table, the bread you bought, etc.). In Portuguese, using articles with common nouns is very frequent, even where English might omit them.

Why does Portuguese say com a faca (with the knife) and not something like “by the knife”?

Com is the normal preposition to express the instrument/tool used to do something:

cortar com a faca = to cut with a knife
You can also say usando a faca (using the knife), but com a faca is the most natural and compact.

Why is there a in a faca instead of uma faca?

A faca = the knife, implying a particular knife (often the one you had in your hand).
Uma faca = a knife, more indefinite/new information (e.g., you grabbed some knife, not previously identified). Both can be correct; it depends on context.

Why does the second part use me: me cortei?

That me is a reflexive/unstressed object pronoun meaning myself. The verb becomes reflexive:

cortar = to cut (something)
cortar-se = to cut oneself
So me cortei literally means I cut myself.

Is me cortei the only possible word order? Could it be cortei-me?

Both exist, but in Brazilian Portuguese, me cortei (proclisis) is much more common in everyday speech and writing.
Cortei-me (enclisis) sounds more formal/literary in Brazil, and less natural in conversation.

Why does Portuguese repeat the verb: cortei ... me cortei?

Because they are two different actions/objects:

cortei o pão (direct object = the bread)
me cortei (object pronoun = myself)
Portuguese normally repeats the verb rather than trying to “share” it the way English sometimes can.

Does me cortei imply it was accidental?

Very often, yes. In everyday use, Eu me cortei typically suggests an accidental cut (e.g., while cooking). If you wanted to sound more explicitly intentional (rare), you’d usually add context.

Why is there a comma before mas?

In Portuguese, when mas (but) connects two independent clauses, a comma is standard:
Eu cortei o pão com a faca, mas me cortei também.

What does também add here, and why is it at the end?

Também = also/too. Here it means: besides cutting the bread, you also cut yourself.
Placing it at the end (me cortei também) is common and emphasizes that the second event happened in addition. You can also say mas também me cortei, which often sounds a bit more balanced.

Could I say Eu cortei o pão na faca instead of com a faca?

Usually no. Com a faca expresses the instrument.
Na faca (literally on the knife) would sound odd and could suggest location/contact in a different way, not the normal “using a knife” meaning.

Is Eu me cortei also correct? What’s the difference from me cortei?

Yes, Eu me cortei is correct. Adding eu makes the subject explicit and can add emphasis/contrast (e.g., “I cut myself (not someone else)”). In many contexts Portuguese drops the subject pronoun because the verb ending already shows who did it, so me cortei is perfectly natural.

How would I make it sound more like “I ended up cutting myself too”?

A very natural option is:
Eu cortei o pão com a faca, mas acabei me cortando também.
Acabei + gerúndio often conveys “ended up (doing something),” especially when it happened unintentionally.