In the first case, you are correct. Both 'so' and 'very' can be used in that case.
However, in the second case, you can also use both 'so' and 'very'. 'The man was so kind to me,' is also correct.
In both the first and the second case, 'very' and 'so' have the same meaning. There is no difference.
In the third case, only 'The man was such a kind person,' is correct. You can also say 'The man was a very kind person.'
'Very' can be used for new information, and 'so' to emphasize it. For example, you could say, 'I'm very tired. I didn't know yoga was so tiring.' In this case, either 'very' or 'so' could be used in both parts, but using 'so' second gives emphasis.
You can also use 'so... that' to emphasize a result. For example, you could say, 'She worked so slowly that she didn't finish in time.' In this case, using 'very' or 'such' would be incorrect.
You can also use 'so' in short answers to agree with someone. For example, you could say, 'I hope so,' in response to a question. In this case, 'very' or 'such' would be incorrect.
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