If you want a line break without having an empty line in between, end the line with two spaces.
This
is
actually
possible.
The text formatting system used on Piefed and Lemmy is called Markup. You can find an excellent short Markup tutorial in the net. It’ll teach the rest.
It appeared in one line because you did not end your lines with two spaces.
You write a line. Then you press the Space bar. Then you press the Space bar again.
And then you press Enter.