O meu irmão fez um arranhão no joelho e a enfermeira pôs-lhe uma ligadura.

In European Portuguese, it is very common to use the definite article before a possessive:

• o meu irmão = my brother
• a minha mãe = my mother
• os meus amigos = my friends

So o meu irmão is the normal everyday way to say it in Portugal.
English does not use an article here, but Portuguese usually does.

Arranhão means a scratch or scrape on the skin.

In this sentence, because it is on the knee and a nurse puts on a bandage, scrape is often the most natural English idea. It suggests a minor injury on the surface of the skin.

Yes, literally fez um arranhão is made a scratch, but in Portuguese this can be a natural way to describe getting or causing a minor injury.

So:

• fez um arranhão no joelho = got a scrape on his knee / scratched his knee

English usually would not say made a scratch here, so this is a good example where you should translate the meaning, not the words one by one.

A more direct alternative in Portuguese would be:

• arranhou o joelho = he scratched his knee

No is a contraction of em + o:

• em o joelho → no joelho

Here it means on the knee or, more naturally in context, on his knee.

This kind of contraction is very common in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

Portuguese often uses the definite article with body parts when the owner is already clear from the context.

So instead of saying:

• no seu joelho = on his knee

Portuguese often simply says:

• no joelho = on the knee / on his knee

Because the sentence is already about my brother, it is obvious whose knee is meant.

This is very common with body parts in Portuguese:

• lavou as mãos = washed his/her hands
• doeu-me a cabeça = my head hurt

Both are in the preterite, the normal tense for a completed action in the past.

• fez = he did / made from fazer
• pôs = she put from pôr

In this sentence, both actions are finished events:

1. the brother got a scrape
2. the nurse put on a bandage

So the preterite is the right tense.

Because that is the correct spelling of the preterite form of pôr in the 3rd person singular:

• eu pus
• tu puseste
• ele/ela pôs

The accent is simply part of this irregular verb form. It helps mark the vowel in the standard spelling.

So you should learn pôs as a whole form, just like you learn fez from fazer.

Here lhe means to him.

It refers to o meu irmão.

So:

• a enfermeira pôs-lhe uma ligadura

literally means:

• the nurse put him a bandage

But in natural English you would say:

• the nurse put a bandage on him
• or the nurse put a bandage on his knee

In Portuguese, lhe is an indirect object pronoun. It can mean to him, to her, or to you (formal), depending on the context.

It refers to the brother, not to the knee.

That is because lhe is being used for the person affected by the action: to him.

The knee is expressed separately with no joelho earlier in the sentence. Even though the second half does not repeat no joelho, the meaning is understood: the bandage is for that injury.

This is a very important feature of European Portuguese.

In affirmative main clauses, unstressed object pronouns often come after the verb, attached with a hyphen:

• deu-me = gave me
• disse-lhe = told him/her
• pôs-lhe = put on him/her

This is called enclisis.

So in Portugal, pôs-lhe is the expected pattern here.

If there is a word that attracts the pronoun, it can move before the verb, for example:

• não lhe pôs uma ligadura = she didn’t put a bandage on him

Here ligadura means a bandage or dressing.

In a medical context, it is something used to cover or protect an injury.

Depending on the exact situation, other Portuguese words can also appear, such as:

• penso = dressing, plaster, bandage depending on context
• adesivo = adhesive bandage / plaster

But in this sentence, uma ligadura clearly refers to something the nurse puts on the scraped knee.

Yes. A few natural alternatives are possible, for example:

• O meu irmão arranhou o joelho e a enfermeira pôs-lhe uma ligadura.
• O meu irmão fez um arranhão no joelho e a enfermeira pôs uma ligadura no joelho dele.

The original sentence avoids repeating joelho in the second half, because it is already obvious where the bandage goes.

That makes the original sentence perfectly natural:

• O meu irmão fez um arranhão no joelho e a enfermeira pôs-lhe uma ligadura.