a) I know you were sixteen on your last birthday.
Or
b) I know you have been sixteen on your last birthday.
Could you please tell me which one is correct? Please give a brief explanation. Thank you in advance.
a) I know you were sixteen on your last birthday.
Or
b) I know you have been sixteen on your last birthday.
Could you please tell me which one is correct? Please give a brief explanation. Thank you in advance.
While this:
I know you were sixteen on your last birthday.
is the more correct of the two, it's a little bit ambiguous, and sounds slightly unusual to my ear; it would be more common to say:
I know you turned sixteen on your last birthday.
I know you were sixteen on your last birthday - is correct.
have been is present perfect and in this context, you are talking about her last birthday, so it does not go that way.
Learn that telling age should be in particular ways. For example -
You were sixteen years old on your last birthday.
You were sixteen on your last birthday
Good reading here.