about what
"О чём Вы думаете?"-"Я думал о вас".
“What are you thinking about?” — “I was thinking about you.”
"О чём ты думаешь?"-"Я думал о Вас".
“What are you thinking about?” — “I was thinking about you.”
"О чём вы говорили?"-"Ни о чём".
“What were you talking about?” “Nothing.”
"О чём вы?"-"Ты прекрасно знаешь, о чём я".
“What are you talking about?” “You know perfectly well what I mean.”