but it don't do no good为什么译文译成“可是一点用也没有”.

but it don't do no good为什么译文译成“可是一点用也没有”.
My wife has given her things,you know,like people do,but it don't do no good.And there's been a lot of sickness around.So we tho't you'd better look her over and tell us what is the matter.
这是自考00600高级英语上册第三课里头一句话.
但是我看着有点儿不懂.
译文：
我太太给她吃了一些药,你知道,大家都是这样做的,可这些药根本不管用,而且,附近有很多人都生了病,所以我们想请您最好给她检查一下,然后告诉我们是怎么一回事.
原句如上,请问
1.it don't do no good是什么句式,为什么译成“不管用”?
2.tho't=thought吗?