What difference is the between "I saw John last week" and " I saw John the last week".
Thanks

The word "the".

"I saw John last week" means you saw him during the week that has passed.

"I saw John the last week" could mean you saw him the last week of his life or the last week of school or the last week before he moved. The sentence isn't a complete thought; a piece of information is missing.

An adverbial phrase answers when or how or why. You are missing part of the when.