难道 and 难怪 — the rhetorical question and the penny dropping

The rule 难道 + question = surely not…? — the speaker already has a view and wants no answer.
难道你不知道吗？ = Surely you know? (meaning: of course you do)
It usually ends in 吗 and carries reproach or surprise.
难怪 = no wonder — said the moment the reason becomes clear: 难怪他没来，原来病了。