加入on 的话就变成强调句式了it is . that ...属于强调句式 去掉的话不会影响句子结构和意义也就是 our class decide to plant trees on April 12th 仍然是完整句式,但是主语从句 不可以 如本句： 如果去掉的话 ...那应该怎么写看你怎样表达了 如下：who decide to plant trees on April 12this our class. - 形式主语： It is our class who decide to plant trees.what our class decide to do on April 12 th is toplant trees. when our class decide to plant trees is April 12th.It is April 12th when our class decide to plant trees.主语不是特别长的情况下没有必要使用形式主语。