I was wondering why there is a difference between the titles for a married and a single woman (Mrs. vs. Miss, respectively), while there is no difference when you refer to a man (both Mr.).
Thank you very much for your help
It's a holdover from a woman's status changing after marriage.
Mr. is an abbreviation of Master.
Mrs. is the abbreviation of Mistress, the corresponding female form. So a Mistress is the wife of a Master.
Miss also comes from Mistress, but a few hundred years ago people began to distinguish between the two, using Miss for unmarried women and Mrs. for married women.
Ms. was adopted as a middle ground between the two, so a woman didn't have to disclose her marital status (and be judged accordingly).