Tense uses and inversion

Do not Quit no matter what… : Don't Quit Don't Quit no matter what! When things go wrong(do we use present simple in when clauses?), as they sometimes will( why do we use "will"?), When the road you're trudging seems all uphill, When funds are low and the debts are high, And you want to smile but you have to sigh, When care is pressing you down a bit, Rest if you must, but don't you quit.(why inversion here?) http://www.english-for-students.com/do-not-quit-no-matter-what.html