关于英语语法问题we will have a picnic in the woods ,weather permitti

关于英语语法问题
we will have a picnic in the woods ,weather permitting.
句子末尾的permit为什么要加ing?

这是独立主格结构.可以当做固定短语去记.
独立主格结构是由名词或代词加上分词构成的一种独立结构,用于修饰整个句子.独立主格结构中的名词或代词与其后的分词等构成逻辑上的主谓关系.这种结构与主句不发生句法上的联系,它的位置相当灵活,可置于主句前、主句末或主句中,常由逗号将其与主句分开.需特别注意的是,独立主格结构与主句之间不能使用任何连接词.
1.作时间状语
My shoes removed,I entered a low-ceilinged room,treading cautiously on the soft tatami matting．
我脱掉鞋子后,走进一间屋顶很低的房间,小心翼翼地踩在柔软的塌塌米垫子上.
The governor pondering the matter,more strikers gathered across his path．
总督思考问题时,更多的罢工工人聚集在他要通过的路上.
2.作条件状语
Weather permitting,they will go on an outing to the beach tomorrow．
如果天气允许的话,他们将在明天组织一次海滨郊游.
Such being the case,you have no grounds for dismissing him．
如果情况如此的话,你没有理由解聘他.
3.作原因状语
The boy leading the way,we had no trouble finding the strange cave.
由那个男孩带路,我们很容易就找到了那奇怪的洞.
There being no further business to discuss,we all went home.
没有别的事可讨论,我们都回家了.
4.作伴随状语或补充说明
I took my ticket,and marched proudly up the platform,with my cheeses,the people falling back respectfully on either side．
我拿着车票还有我的奶酪,雄赳赳气昂昂地跨步走向月台.人们似乎很尊敬我,纷纷向两边退去.
Ten students entered for the competition,the youngest a boy of 12.
十个学生报名参加了这次竞赛,年纪最小的是个12岁的男孩.
5.作定语,独立主格结构作定语其功能相当于一个定语从句.
He is the person with a lot of questions to be settled.
他就是有许多问题要解决的那个人.
He was walking along the road without any street lights on its both sides.
他走在一条两边没有路灯的马路上.
Close to the bank I saw deep pools,the water blue like the sky.
靠近岸时,我看见几个深池塘,池水碧似蓝天.