我认为这主意不错但她不同意用英语怎么说

你可以说 "I think it's a good idea, but she doesn't agree."

"I think" 是表达自己主观意见的常用词汇，"but" 是连接句子的连词，表示转折关系，"she doesn't agree" 表示对方不同意。

例句：

I think we should go out for dinner, but my wife wants to stay home.
(我认为我们应该出去吃饭，但是我妻子想呆在家里。)
I think this plan is solid, but my boss has some concerns.
(我认为这个计划很可行，但是我的老板有些担心。)
I think this stock is a good buy, but my financial advisor disagrees.
(我认为这只股票是个好买卖，但我的财务顾问不同意。)