Musisz tego samego czasu używać:
I'm watching TV. So am I.
I went to the movies yesterday. So did I.
I have seen that movie already. So have I.
He is watching TV. So is she.
She cooks well. So does he.
W potocznej mowie można po prostu powiedzieć "me too" zamiast "so do I", "so have I", "so did I".