Eu cortei o dedo quando usei a faca.

With body parts, Portuguese often uses the definite article instead of a possessive when it is already obvious whose body part it is.

So Eu cortei o dedo naturally means I cut my finger in context.

Using meu dedo is possible, but it is usually more emphatic or contrastive, as if you wanted to stress my finger, not someone else’s.

Cortei is the pretérito perfeito of cortar.

It means a completed action in the past:

• eu corto = I cut / I am cutting
• eu cortei = I cut

So Eu cortei o dedo describes something that happened and was completed.

Yes. Portuguese often drops subject pronouns because the verb ending already shows the person.

So both of these are correct:

• Eu cortei o dedo quando usei a faca.
• Cortei o dedo quando usei a faca.

Including eu can add emphasis, clarity, or contrast, but it is not required here.

Because the sentence is talking about a past situation, and usei is also in the pretérito perfeito:

• usei = I used

So quando usei a faca means when I used the knife.

Using the preterite here presents that action as a specific completed event. The sentence treats both actions as past events:

• I used the knife
• I cut my finger

Here quando means when.

It connects the action of cutting the finger to the time of using the knife:

• Eu cortei o dedo quando usei a faca. = I cut my finger when I used the knife.

However, in natural English, we might sometimes say while using the knife. In Portuguese, if you want to emphasize an ongoing action more clearly, you might hear:

• Eu cortei o dedo quando estava usando a faca.
• Eu cortei o dedo usando a faca.

So quando is correct, but other structures can sound more natural depending on the exact nuance.

Portuguese uses definite articles more often than English.

So a faca means the knife, but in many contexts English would simply say a knife or just use the noun more loosely.

Here a faca sounds normal in Portuguese because it refers to the knife involved in the situation. It does not necessarily have to mean a very specific knife already known to the listener the way English the knife often does.

Yes, it is understandable and grammatical. But depending on what you want to emphasize, many Brazilians might say one of these instead:

• Eu me cortei com a faca. = I cut myself with the knife.
• Eu cortei o dedo com a faca. = I cut my finger with the knife.
• Eu cortei o dedo usando a faca. = I cut my finger while using the knife.

Your sentence, Eu cortei o dedo quando usei a faca, is fine, but it can sound a little more like I cut my finger when I used the knife than while I was using the knife. The alternatives above may sound more natural in everyday speech depending on context.

Both patterns are possible, but they focus on different things.

• Eu me cortei. = I cut myself.
• Eu cortei o dedo. = I cut my finger.

With me cortei, the focus is on the person being injured. With cortei o dedo, the focus is on the body part.

You can also combine ideas in other natural ways:

• Eu me cortei no dedo. = I cut myself on the finger.
• Eu me cortei com a faca. = I cut myself with the knife.

Yes. A few common alternatives are:

• usando a faca = using the knife
• quando estava usando a faca = when I was using the knife
• com a faca = with the knife

Examples:

• Eu cortei o dedo usando a faca.
• Eu cortei o dedo quando estava usando a faca.
• Eu cortei o dedo com a faca.

Each version gives a slightly different nuance:

• quando usei = when I used
• quando estava usando = when I was using
• usando = while using
• com a faca = with the knife / by means of the knife

Yes. Portuguese allows some flexibility.

For example:

• Eu cortei o dedo quando usei a faca.
• Quando usei a faca, cortei o dedo.

Both are correct. The second version puts more focus on the time clause first: When I used the knife, I cut my finger.

In Brazilian Portuguese, cortei is pronounced approximately like cor-TAY.

A rough breakdown:

• cor sounds like English core, but shorter
• tei sounds like tay

And usei sounds approximately like oo-ZAY.

These are only rough English approximations, but they can help you get started.