Learn Hardcore Portuguese (Portugal): O desconto foi aplicado automaticamente quando paguei com cartão. - The discount was applied automatically when I paid with a card.

The speaker wants to emphasize the discount rather than who applied it. In Portuguese the passive voice is formed with the auxiliary ser (here in the preterite, foi) plus the past participle (aplicado). It corresponds to “the discount was applied.”

How do you form the passive voice in Portuguese, and why does aplicado agree with desconto?

You combine the correct form of ser with a past participle. The participle must agree in gender and number with the subject: here desconto is masculine singular, so we use aplicado, not aplicada or aplicados.

Why is the simple past paguei used instead of a present perfect like in English (“I have paid”)?

In European Portuguese, finished actions are normally expressed with the simple past (pretérito perfeito), not the present perfect. So “I paid” → paguei, even if in English you’d say “I have paid.”

Could you say ao pagar com cartão instead of quando paguei com cartão, and what’s the difference?

Yes.

Quando paguei com cartão uses a finite verb in past tense.
Ao pagar com cartão uses the preposition ao
- infinitive to convey “upon paying.”
  They’re interchangeable in meaning, though ao + infinitive is slightly more formal or written.

Why is there no article before cartão (i.e. com cartão and not com o cartão)?

When talking about means of payment in a general sense, Portuguese often omits the article, just as English does with “with card.” If you want to specify your card, you could say com o meu cartão, but it isn’t necessary here.

Is it okay to move the adverb automaticamente elsewhere in the sentence?

Yes. Adverbs of manner are flexible. You could say:

O desconto foi automaticamente aplicado… (between auxiliary and participle)
O desconto foi aplicado automaticamente… (after the participle, as in the original)
Both are correct—choice depends on emphasis or style.

Can you use an active construction instead, like Aplicaram-me o desconto?

Yes. That’s a pronominal passive/impersonal construction:

Aplicaram-me o desconto quando paguei com cartão.
Here me is the indirect object “to me,” and the verb stays in the 3rd person plural. It’s more colloquial, but perfectly correct.

What tense is foi aplicado exactly?

It’s the pretérito perfeito composto of the passive voice? No—it’s simply the pretérito perfeito simples of ser (“foi”) + past participle. In English: “was applied.”