You should always use a hard return (Enter key) whenever you complete a paragraph. I still see many documents where people type a hard return after each line like they would on a typewriter. Word knows by the margins set in a document when to wrap the text to the next line and uses soft returns to do this. Soft returns will change position automatically as you add or delete words or when you change margins. You can enter a soft return (Shift+Enter) yourself if you want to create a new line but stay in the same paragraph. Paragraph styles are applied within a paragraph, defined by any line(s) of text followed by a hard return. If you use a heading style but want to break up the heading on two lines use a soft return.

(To toggle the display of non-printing characters press Ctrl+*)

But if you have full justification and end your paragraph with a soft return this is what happens to the last line.

When you want to end a line before you reach the right margin (like at the end of the paragraph above), use the hard return. You want to force a return after a heading or title, or when you’re entering lines of an address. In these cases you should generally use a hard return.