"Helen used to write" is more natural. "Would" indicates a habit or a state in the past which may or may not continue now. The focus is on the past. There is not usually a comparison with the present. Last year, my brother would walk to work. (no reference to the present - we don't know if he still walks to work) Last year, my brother always walked to work. (no reference to the present - we don't know if he still walks to work) Last year, my brother always walked to work, but now he drives to work. (natural - reference to the past and the present) Last year, my brother would walk to work, but now he drives to work. (unnatural because "would" is not normally used to show contrast) "Used to" indicates a habit in the past that no longer happens. The focus is on a comparison between the past and the present. Last year, my brother used to walk to work, but now he drives to work. (natural - reference to the past and the present) Last year, my brother used to walk to work. (but doesn't now) (natural - reference to the past and implied reference to the present)

Yes, that's fine. Take a look at this if you'd like to do some more exercises: - https://www.examenglish.com/grammar/used_to_would.htm Hope that helps.