How will be more correct to say negative sentense with "used to"? In textbooks there are examples where "ed" disappears, but I heard that people can use with "Ed". For example, I didn't used to go to the beach when I was a child

See a translation

I didn't used to go to the beach when I was a child.

Perfectly natural and native speaker sentence right there. Good job.

To make a negative sentence with "used to," using "never" or "didn't" is an easy and common way to make it work.

We never used to speak to our parents like that.

We didn't used to drink whiskey before class.

Did you use the pencil I gave you?

In spoken language, at least in my dialect, "used to" sounds exactly like "use to," where the "s" has a proper "s" sound. When we say "use" the "s" turns into a "z."

I'm from the San Francisco area in California.