如果你不同意某个观点或者想法，可以说 "I don't think so" 或者 "I disagree." 来表达你的不同意。这两个短语都来源于英语中的否定形式，表示不同意对方的观点或想法。

例句：

"Do you think we should go to the party tonight?" "I don't think so. I'm really tired." (你认为我们今晚应该去参加聚会吗？" "我不这样认为。我真的很累。")

"Do you think that climate change is a hoax?" "I disagree. The evidence shows that it is real and happening." (你认为气候变化是一个骗局吗？" "我不同意。证据表明它是真的，正在发生。")