Hi Enrico, I don't think I fully understand the question, but maybe I can help a little bit. You wanted to know how to write two sentences. One for a situation in which you frequently steal pens and another in which you stole a single pen. Is that right? When you write that Mary "blamed" you, do you mean "blamed" or "accused"? There are many ways to write the sentences, but here are two possibilities.
1. I (always, often, sometimes, frequently, usually) steal pens from the office. Mary blamed me for stealing the pens.
2. I stole a pen from the office and Mary blamed me for it.
In the second sentence, "a pen" shows that a single pen was stolen.
Does that help at all?
Student or Learner