Question on modal verbs

Question

Can we form the negative form of this sentence, she used to swim on Sunday’, in the following ways? 1) She doesn’t use to swim on Sunday. 2) She didn’t use to swim on Sunday.

In thw first sentence why do we use infinitive form of ‘use’ with doesn’t, even if ‘used to’ is semi modal?

Answer 1

This is a valid negative:

She did not use to swim on Sundays, but now she has taken up the practice.

You could also say

She never used to swim on Sundays, but now she has taken up the practice.

Your version with does is not a valid tense for referring to the past.