I worked here before means that I worked there at a definite time in the past and I don't work there any more. I have worked here before troubles me because the adverb before put the action of the subject 'I' in the past, so there is some sort of incongruity of using the present perfect tense jointly with the marker of closed past, the adverb before .
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Confusant1Is there a difference between those two?You didn't include the sentences you were asking about in the body of the post, so it's not possible to use the quote button to quote them in the answer. You might consider organizing your posts so that the relevant examples can be quoted.