不用穿羽绒服---no need to put on..because it isn't cold 不要穿羽绒服---do not put on..most times it means "do not put on the down jacket, put on other clothes"; sometimes, it means "should not put on" 不得穿羽绒服---can't put on..most times, it means "you can't put on the down jacket, but you can put on other clothes". That is, putting on the down jacket is not allowed. the difference between the 2nd one and the 3rd one is TONE.

不用穿means needn't 不要穿means don't The correct is "不得不” not “不得”. “不得不”means have to do something

不用穿羽绒服is right!

天气 不 冷， 不用 穿 羽绒服。