You can use "do" in affirmative clauses for emphasis.
My mother thinks I don't like studying history, but I DO like it; the truth is that I don't like my history teacher.
Mrs Maylie thinks her husband doesn't love her any more, but he DOES (no "s") love her.
DO come in. (instead of "Come in"); DO have a good cup of tea. (instead of "Have a cup of tea")
You can use "did" in the same way.
My mother thought I didn't like studying history when I was a child, but I DID like (not "liked") it.
ok, I understand what you are explaing me. I misunderstood with the clause: Mrs Maylie thinks her husband doesn't love her any more, but he DOES (no "s") love her. where (no "s") is referred to love and not to "does".
Thank you for you explanation