I am using Word 2012 (yes an oldy but it does everything I want it to do).

Sometimes when I 'delimit' a line by pressing SHIFT + ENTER key and then use the TAB key so that the line starts to the right of the start of the previous line, the previous line also gets moved to the right. I hope I have explained what is happening for you to understand.
What causes this and how do I prevent it happening?

The quick brown dog <this line also moves to the next tab position)
(tab) jumps