"The amusement park had closed."
Translation:Nöjesfältet hade stängt.
So is there a nuance to help understand where you must use stänga and where sluta?
stänga when you could say close in English and sluta when you could say quit in English.
There's also another verb sluta which means 'close', but it's more formal and basically only used in set expressions these days. This verb has the present tense sluter and the past these slöt, slutit but the sluta that means 'quit' is slutar, slutade, slutat.