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 sb (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 sb against doing sth, 表示“告诫某人不要做或提防某事”（常可与 warn sb 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 sb+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 sb+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
