Line break problem

When i edit text and add two spaces after the end of a line, i get a line break (
) in the html output. But if i need a second
and try it with two additional spaces are two spaces after the return, it did not work. Only manual adding
in the text works. But this is not a fine solution. I know such tags, but the end user for the website are not able to using html syntax later and this additional
makes the text not so readable.

Is this Markdown you are editing? If so it should output paragraphs, not line-breaks, which you should style with CSS. Whilst double-spaces can create a line-break, it is not advisable for just the reason you outlined: You end up relying on manual formatting occasionally.

As Grav uses GitHub-Flavored-Markdown through Parsedown, you can use regular line-breaks (not HTML, just line skipping) to achieve the desired separation of single and double line-breaks (see cheatsheet).