Hello and Good Afternoon,
Thank you for asking this question about the when and where to use the the second person singular. In response to your question, it all depends on who you are talking to and the environment you're in when holding a conversation with another person.
If you know the person you are speaking to, perhaps on a more 'familiar' basis (as in a family member or a very close friend), then you can speak to that person using the second person singular format.
If you have never met this person before or if you are in a business environment, then it would be proper to use the third person singular format, or a more 'polite' format. Please note that if that person gives you permission to use the second person singular format, then you can switch over; otherwise, I would stick to the third person singular format.
BTW ... when using 'lei' in a polite format, you'll need to capitalize the 'L' so as to differentiate this from the third person feminine format. (ex. darsi del Lei).
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