SentenceForm form = new SentenceForm();   

form.MdiParent = this;   

form.Show();

//form.MdiParent = this;用了这句代码就只能form.Show()   

//form.ShowDialog(this);//要用这句代码就得注释掉form.MdiParent = this;

2012-4-2 通过MdiParent设置窗体最前