It can be a little confusing because "closed" can function as a verb or an adjective, but I'll focus primarily on its use as an adjective. "The shop is closed" means it is closed right now or at a specified time. The shop is closed on Sundays. The shop is closed between 9pm and 9am. "The shop was closed" means it was closed at some point in the past. The shop was closed last night. "The shop had closed" is used when you're talking about more than one event in the past, and one event occurred before the other. I wanted to buy some milk last night, but the shop had closed(before I arrived). "Closed" can only be a verb in this case. "The shop has been closed" means the shop closed at some point in the past and is still closed. The shop has been closed since Sunday because the owner is on vacation. "The shop had been closed" is used when the shop was closed in the past, but is now open. The shop had been closed for renovations, but reopened today. Some of these can be interpreted as the passive form of the verb describing the action of closing the store, either for the night, or permanently. The shop was closed by the owner.

You can look up these grammar tenses: Present simple Past simple Present perfect Past perfect Present perfect passive Past perfect passive You need to practice these properly, and this takes time. This isn't something you suddenly know nor learn quickly.