As Wyt mentioned in Shakespeare's proverb from the play A Midsummer Night's Dream it says : The course of true love never did run smooth , yet to use 'smooth' as an adverb ,that is only a slang or in dialectic speech. In TV series ,they use a lot of slang , but it is not grammatically correct and you can not use it as an adverb, while writing a comprehension or official documents etc .. So the correct adverb is 'smoothly' ,run smoothly ! Always distinguish between the slang used on TV and correct English ! You can use the slang in your speech ,but learn the grammar rules to apply at work and in exams ( if you are taking any ) ,etc .... 加油：） 我在學習中文，你可以幫助我嗎？ ：） 謝謝

此处句子的主语true love是形容词smooth的逻辑主语，它表示主语的状态或结果。而非修饰run这个动词的程度或过程 The course of true love never did run smooth这句是仲夏夜之梦里的与此类似

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