Learn Finnish: Se ei ole fakta, vaan vaarallinen harhakäsitys. - It is not a fact, but rather a dangerous misconception.

Both words translate to 'but', but they are used differently. We use vaan after a negative clause (like Se ei ole fakta) to offer a direct replacement or correction, meaning 'not X, but rather Y'. If you were just adding contrasting information without correcting the first part, you would use mutta.

Since this is a negative sentence (ei ole), why aren't the words fakta and harhakäsitys in the partitive case?

When saying what a singular, countable thing is or isn't using the verb olla (to be), the noun usually stays in the nominative case. If we were talking about an uncountable substance, it would take the partitive (for example, Se ei ole vettä - It is not water). Since a fact and a misconception are distinct, countable concepts, they stay in their basic nominative forms.

How is the compound word harhakäsitys formed?

It is made of two words: harha (an illusion, delusion, or stray) and käsitys (an understanding, conception, or impression). Together, they literally mean an illusory or false understanding, which gives us 'misconception'.

Is the comma before vaan strictly necessary here?

Yes. Finnish punctuation rules are quite strict about this. You generally must place a comma before the conjunctions mutta and vaan, even when they just introduce a contrasting phrase rather than a full independent clause.