Johanna - are you by any chance hitting the carriage return at the end of the line ? I find it helps to actually show carriage returns, tabs etc in Word, so that I can see where they are. Word should automatically *wrap* around within the margin limits, you shouldn't need to hit any tab keys whatsoever.
The carriage return key is on the *Standard* Toolbar (to see this one, go to View, Toolbars and Click against Standard), 4th button from the right, left of the 100% button and click on it, to *show* it.