think在否定句前加don't，因为英语动词否定需要变成do-support结构。比如"I don't think"比"I think not"更自然，就像"I don't want"比"I want not"一样。这种结构让句子更符合英语习惯，也方便后续加其他成分。

英语否定句要求及物动词必须搭配助动词do，这样not才能正确修饰动词原形。数据显示80%的及物动词需要do-support，比如go→don't go，want→don't want。如果直接加not，比如"I think not he is here"，会显得生硬，因为not只能修饰动词原形，不能直接跟宾语。而don't think he is here，not放在助动词后，既符合语法规则，又让句子更流畅。比如否定前移后，可以连续加其他成分："I don't think he is here, and she doesn't either." 这样not和助动词组合，能更灵活地扩展句子。但要注意not不能放在句尾，比如错误句"I think he isn't here"虽然正确，但not在句尾时，通常用其他结构表达，比如"I think he isn't here, but he might be.