1. 用於 warn sb (not) to do sth, 表示“告誡某人（不）要做某事”。如：He warned me not to touch it. 他警告我不要觸碰它。Ann warned me to read the instructions. 提醒我看看說明書。
2. 用於 warn sb against doing sth, 表示“告誡某人不要做或提防某事”（常可與 warn sb not to do sth 互換）。如：They warned him against swimming [=not to swim] in the lake. 他們告誡他不要在這湖裡游泳。
3. 用於“warn sb+that-從句”，表示“警告某人……”。如：He warned me (that) f was in danger. 他警告我處境危險。I have warned him that it is not allowed. 我警告過他這是不允許的。
4. 用於“warn sb+介詞(against, about, of) +名詞”。如：He warned her against pickpockets. 他讓她提防扒手。I warned him about those stairs. 我警告過他要注意（留意）那些樓梯。forget的句型forget doing是正確的,忘記做過某事,事情已經做過了forget to do 也是正確的,意思是忘記去做某事,事還沒做
warn的句型：用作動詞，表示“警告”“告誡”，注意以下句型和結構：
1. 用於 warn sb (not) to do sth, 表示“告誡某人（不）要做某事”。如：He warned me not to touch it. 他警告我不要觸碰它。Ann warned me to read the instructions. 提醒我看看說明書。
2. 用於 warn sb against doing sth, 表示“告誡某人不要做或提防某事”（常可與 warn sb not to do sth 互換）。如：They warned him against swimming [=not to swim] in the lake. 他們告誡他不要在這湖裡游泳。
3. 用於“warn sb+that-從句”，表示“警告某人……”。如：He warned me (that) f was in danger. 他警告我處境危險。I have warned him that it is not allowed. 我警告過他這是不允許的。
4. 用於“warn sb+介詞(against, about, of) +名詞”。如：He warned her against pickpockets. 他讓她提防扒手。I warned him about those stairs. 我警告過他要注意（留意）那些樓梯。forget的句型forget doing是正確的,忘記做過某事,事情已經做過了forget to do 也是正確的,意思是忘記去做某事,事還沒做