Is there any connection between arfer (here rendered as 'used to') and ymarfer which appears elsewhere in the course with the meaning 'practice' or 'exercise'? The connection could be that, if one used to do something, one was making it one's practice to do so. Or is the resemblance purely coincidental?
The use of arfer here means being in the habit or custom of doing something, being used to doing something.
When used with the imperfect tense, as in the example, the combination is usually translated into English as 'used to'.