If you want to express a change of tone, you can use 그러나, 그렇지만, 하지만, 그런데, 근데 when the next words will be in conflict with the previous content or opposite. Personally, I think 그러나, 그렇지만, and 하지만 can be grouped together, and are used when the content of the preceding and following text is opposite or opposing. 그런데 and 근데 can be used not only as 'but', but also to change the topic to another topic, similar to 'in addition, ...'.
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