she wanted to know后面是该接 i thought i twas not a good idea还是i didn't think it was a good idea

选择后面那种 即：I didn't think it was a good idea.（我认为那不是个好主意）
为什么呢?这涉及到否定转移的知识.
像think,believe,suppose引导的宾语从句里常把从句的否定词转移到主句中：
例如：我认为你是错的!
正确写法：I don't think you are right.（我认为你是错的）---不可翻译为：我不认为你是对的.（虽然意思差不多,但很绕口,绕着绕着人就晕了）
所以我们在翻译或理解句意时,要知道I don't think的not 是属于are not right的.
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接着看你的那道题：
我认为那不是个好主意：
i thought it was not a good idea------中文意思可以这样理解
但老外在书写时,要否定词（这里是not）转移.老外会写成：
I didn't think it was a good idea.i thought it was not a good idea这个算中文式英语吗？对，中式英语！老外是不会这么写的。