Student or Learner
1- Clare began to think over her teacher Irma who tought English at that time.
2- Clare began to think over her teacher Irma, who tought English at that time.
Here is my interpretation:
1- Without the comma before 'who', what is assumed is that that Clare had two teachers named Irma and they were both English teachers.
2- With the comma, there was only one whose name was Irma.
I'd like to ask if my interpretations, especially one, are OK or not.
Neither of these sentences are natural. You also mispelled 'taught'. I don't know what you mean by 'think over'.
Regardless, there's nothing in either sentence to suggest there are two teachers. Both clearly state that there's a teacher named Irma and a student named Clare, and that's it.
Wear short sleeves! Support your right to bare arms!