warn的句型：用作动词，表示“警告”“告诫”，注意以下句型和结构：

1. 用于 warn *** (not) to do sth, 表示“告诫某人（不）要做某事”。如：He warned me not to touch it. 他警告我不要触碰它。Ann warned me to read the instructions. 提醒我看看说明书。

2. 用于 warn *** against doing sth, 表示“告诫某人不要做或提防某事”（常可与 warn *** not to do sth 互换）。如：They warned him against swimming [=not to swim] in the lake. 他们告诫他不要在这湖里游泳。

3. 用于“warn ***+that-从句”，表示“警告某人……”。如：He warned me (that) f was in danger. 他警告我处境危险。I have warned him that it is not allowed. 我警告过他这是不允许的。

4. 用于“warn ***+介词(against, about, of) +名词”。如：He warned her against pickpockets. 他让她提防扒手。I warned him about those stairs. 我警告过他要注意（留意）那些楼梯。forget的句型forget doing是正确的,忘记做过某事,事情已经做过了forget to do 也是正确的,意思是忘记去做某事,事还没做。