Learn Russian: Сыровар вернулся в погреб спустя год. - The cheesemaker returned to the cellar a year later.

Both mean 'a year later', but спустя is used almost exclusively for looking back at a sequence of events in the past. It gives a narrative or storytelling feel. Через is much more versatile and is the standard way to say 'in a year' for future events, though it can also be used for the past.

Could we change the word order to год спустя?

Yes! The preposition спустя is quite unique in Russian because it can be placed either before the time period (спустя год) or after it (год спустя). Both are grammatically correct and mean the exact same thing, though placing it after the noun is very popular in literature.

The lesson says спустя takes the Accusative case, but год looks like the Nominative case here. Why?

The word год is indeed in the Accusative case here. Because it is a masculine, inanimate noun, its Accusative form is identical to its Nominative dictionary form. If the time period were feminine, like a minute (минута), you would clearly see the Accusative change: спустя минуту.

Why is it в погреб (Accusative) rather than в погребе (Prepositional)?