Angular1.X中遇到的问题

在form表单中，用ng-if时下面的ng-model无效；换成ng-show之后就神奇般的好了。求大神解释！代码见图。

It is because "ng-If" differs from "ng-Show". "ng-If" completely removes and recreates the element in the DOM rather than changing its visibility via the display css property.