Compare:
She sat in the armchair. (In – preposition; armchair – object)
Please come in. (In – adverb; no object)
He stood before me. (Before – preposition; object – me)
I have seen him before. (Before – adverb; no object)
She put the book on the table. (On – preposition; object – the table)
Let’s move on. (On – adverb; no object)
He will return after a month. (After – preposition; object – a month)
He came soon after. (After – adverb; no object)