Et maintenant il ne se brosse pas du tout les dents...... parce qu'il n'a plus de dents
He used to only brush his teeth once per month - rejected :
I have to assume all these sentences are about Duo's ex Thomas.
Well, In my years here I never heard that Duo had a relationship with anybody in particular. Where did you get this Thomas from? Is Duo a boy?
one time per month/ once a month/once per month are all good
English translations of the French phrase
"He used to brush his teeth only one time per month" - rejected why? (and I'm a native English speaker)
Same question, he used to only brush his teeth once a month. Complete guess work as to whether or not Duo wants us to use 'used' or not.
He didn't use to brush his teeth but once a month - why not? Or He didn't brush his teeth but once a month - why not?
Because it's the opposite of what you're trying to say (a double negative): "He would (or used to) brush his teeth but once a month.".