This is the 15978th most frequent Russian word.
прошедший
"Прошедший" means "past" or "elapsed."
In this example, 'прошедший' is a participle from 'пройти,' implying 'the one who passed.' It refers to a person or subject who completed the action of passing through the forest.
Прошедший через лес оказался на поляне.
The one who passed through the forest found themselves in the clearing.
The word 'прошедшем' is the prepositional case of 'прошедший,' used here as a participle to describe the noun 'событии,' indicating the event's occurrence in the past.
Он рассказал нам о событии, прошедшем неделю назад.
He told us about the event that took place a week ago.
Here, 'прошедший' is used as an adjective derived from the verb 'пройти,' meaning 'past' or 'previous.' It describes the noun 'год' as the one that has passed.