Sooo what I’m doing now because I hate Tumblr.
Either shift + enter to get to a new line

Or twice to get back to paragraphs.
Alternatively you can go into the html and use <br> for new lines.

If you want to put a photo in but you’ve gone down a few lines use;
ctrl + shift + P and it will bring up the menu when you normally press the + on a new line.

Alternatively shortcuts are as such for; this line;   ctrl + shift + l

and for a readmore it is ctrl + shift + k

I think that’s all !