There are lots of rules. Years ago, I wrote a lengthy reply to a question like yours, and I used to be able to link to it by post number when similar questions about the position of adverbs were asked. This is no longer possible, so I'll just say that in your first sentence suddenly is indeed after the verb, but it's more useful to think that it is at the end of the sentence because that is one of the three most common positions for adverbs in a sentence.
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