explanation please

[Vala]

I started to learn estonian from textbook TEACH YOURSELF ESTONIAN and sometimes it is very confusing. Because they use more forms for translation of one english sentence without any explanation why they choose this and not that form.
Could you please help me with my confussion?

Which is the right form to use when you say: ""What is your first name?" ?
"KUIDAS on teie eesnimi?" or"MIS on teie eesnimi?"
Or both can be used?

[ainurakne]

Literally "Mis on teie eesnimi?" means "WHAT is your first name?"
and "Kuidas on teie eesnimi?" means "HOW is your first name?"

But they are used pretty much interchangeably.

[Vala]

I thought that it could be so.
Thank you for confirmation