- I study French every Monday with a tutor.
- Right now, I'm not studying French.
That example above is what is usually meant by the "right now" rule.
However, please consider:
- She studies French at university.
- She is studying French at university.
Question, implicit or actual: What does she do? [in life].
Answer: She studies French at university.
Question: What is she doing? [at this point in her life].
Answer: She is studying French at university.
Answer: The choice of simple present versus present continuous depends on the implicit or the explicit question or idea being expressed. Are you expressing a general idea (study French) or are you expressing a specific activity at a specific time? (studying French)
[Please note: The gerund structure, "studying French" is a separate issue.