Learn Swedish: Jag är arkitekt, men jag är dålig på att måla. - I am an architect, but I am bad at painting.

In Swedish, you generally drop the indefinite article (en or ett) when stating someone's profession, nationality, or religion. You just say "I am architect" (Jag är arkitekt). This "zero article" rule is very similar to how Swedish drops articles for mass nouns in general statements.

Why do we use på after dålig?

In English you say you are "bad at" something, but Swedish uses the preposition på (on). The phrases bra på (good at) and dålig på (bad at) are fixed expressions that are very common and should be memorized together.

The English translation says "painting", so why does the Swedish sentence use att måla ("to paint")?

Swedish doesn't have an "-ing" form (gerund) that acts like a noun. When you want to use an action right after a preposition like på, you use the infinitive marker att followed by the base verb (måla). Literally, it translates to "I am bad at to paint".

Would dålig change if the subject was plural, like "we"?

Yes! Because dålig is an adjective describing the subject (jag), it must agree in number. If the sentence was "We are bad at painting", you would add an -a to make it plural: Vi är dåliga på att måla.