这个问题其实很简单：
1 如果你是要是否定整个句子,就在动词前面加助动词.
2 如果你是要否定不定式的动作,就在不定式符号to前面加not.
3 有些否定转移的动词像think,believe,suppose,imagine,expect等后面接宾语从句,要对从句进行否定,该否定应移至主句,即否定主句的这些谓语动词.
I don't think it will rain tomorrow.
We don't expect our football team will win the World Cup.
欢迎探讨词法语法问题.