Learn Dutch: Het café is niet open, maar dicht. - The cafe is not open, but closed.

We use niet because we are negating the adjective open. You only use geen to negate a noun (like saying not a or no in English). Since open is an adjective describing the cafe, niet is the correct choice.

Does dicht mean the same thing as gesloten?

Yes, both mean closed. Dicht is very common in everyday spoken Dutch for shops, doors, or windows being shut. Gesloten is a bit more formal, like the official closed sign you might see on a shop window.

Why does café have an accent mark on the e?

Dutch borrows this word directly from French. The accent tells you to pronounce the e as a long ay sound, rather than the short uh sound a single e usually makes at the end of a Dutch word.

Is it required to put a comma before maar?

Usually, yes. In Dutch, it is a standard rule to put a comma before the conjunction maar when it connects two contrasting ideas or phrases.