warn用作动词，表示“警告”“告诫，提醒”等，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。

《英语常见问题解答大词典》赵振才P1543,明确答复没有warn to do用法，大家注意，但是有warn sb (not) to do sth，这个用法还可以于被动语态。

The patient was warned not to eat oily food after the operation.=

The doctor warned the patient not to eat oily food after the operation.

那个病人被告诫在手术后不要吃油腻食品。

They were warned not to climb the mountian in such bad weather.

有人劝诫他们不要在这么坏的天气去登那座山。

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，off) +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。

注意，warn习惯上不能后接双宾语，即不用于warn sb sth结构，可考虑改用warn sb of sth。如：

我们警告过他有危险。

误：We warned him the danger.

正：We warned him of the danger.

I am warning you off my land.

我警告你离开我的土地。

5. warn只用作动词，不用作名词，其相应的名词是 warning。如：

They have been given a warning. 他们受到警告。

He would go in spite of our warning. 尽管我们提出警告他还是要去。