Why does using makes my text go to the next line?

I am just a beginner, so this might be a stupid question. I wanted to make few words in my text bold by adding but when I do it the text goes to the next line and it looks like this:

Lorem ipsum dolor sit amet, consectetuer adipiscing elit.

Nulla

pulvinar eleifend sem. Aenean vel massa quis mauris vehicula lacinia. Maecenas aliquet accumsan leo. Nullam eget nisl. Pellentesque

pretium

lectus id turpis. Integer lacinia. Mauris dolor felis, sagittis a

Anyone know how to solve it?