Learn Danish: Døren åbnes ved hjælp af et kort. - The door is opened by means of a card.

Both forms mean 'is opened', but the s-passive (åbnes) is preferred for general facts, instructions, or ongoing rules. The blive-passive (bliver åbnet) is typically used for a specific action happening at a particular moment. Because this sentence explains the general mechanism of how the door works, the s-passive is the most natural choice.

Could I just say med et kort instead of ved hjælp af et kort?

Yes, med et kort (with a card) is grammatically correct and very common in everyday speech. However, ved hjælp af (by means of) sounds slightly more formal and technical. This makes it pair perfectly with the formal instructional tone of the s-passive verb åbnes.

Why is there no article before the noun hjælp in this phrase?

In Danish, many fixed prepositional expressions drop the article before the noun. Even though you would normally say en hjælp (a help) in an ordinary sentence, this specific set phrase is always strictly ved hjælp af, translating literally as 'by help of'.

How does the word order change if I start the sentence with the method?

Moving the prepositional phrase to the front to emphasize it triggers the V2 rule, meaning the verb must remain the second main element of the sentence. The subject (døren) and the verb (åbnes) will swap places, resulting in: Ved hjælp af et kort åbnes døren.