Why is the word 'off' sometimes used, as in the following:<br/><br/>"You will have to <b>learn</b> it <b>off</b> by heart", "Let's <b>start off</b>".<br/><br/>What is the difference between that and just saying "learn\start".<br/><br/>When should (or could) 'off' be added?<br/><br/>Many thanks,<br/><br/>D.A.