英语：副词slowly的比较级为什么是more slowly 而不是去y为i加er?

英语：副词slowly的比较级为什么是more slowly 而不是去y为i加er?
slowly的音标[ˈsləʊli] 是双音节,而且以“辅音+y”结尾的吗 ,不是应该去y为i加er?

大家都各有看法,来听听外教是怎么说的吧：
Reference to the rules of grammar helps clarify this topic.
对待这个问题应当参照语法
1.The comparative form of regular adjectives is made by adding -ER to the word (and the superative form by adding -EST.) But this rule can be safely applied only to monosyllables.
1b.Rule 1 also applies to adverbs (or adjectives that are used like adverbs) e.g.FAST,FASTER,FASTEST.
一般形容词的比较级是在其后加ed（最高级加est）,但是前提是单音节词.本条同样适用于副词.
【2.Many adjectives with two syllables e.g.BURLY e.g.ORANGE do not follow rule 1.Their comparative forms must be made by the word MORE as MORE ORANGE,
MORE BURLY etc.No one allows ORANGER;
BURLIER may be allowed (but users disagree,some
preferring MORE BURLY.)】
【很多双音节形容词,如burly、orange不遵循上一条,他们的比较级是加more.】
【3.Adverbs usually end in -LY i.e.have two or moresyllables,thus follow Rule 2 rather than Rule 1.】
【以ly结尾的双音节或多音节副词更多的是遵循第二条而不是第一条.】
SLOWLIER is the application of Rule 1 to SLOWLY,
contrary to Rule 3 and is an error.This general type is common,following (Rule ) the guideline that,when in doubt,we should treat words as regular
rather than irregular.This is probably what prompts young children to use GOOD,GOODER,GOODEST
【Slowlier违背了第三条,错误地去遵循了第一条.】这种情况很常见我们应当遵照语法来,这就是为什么很多孩子犯good,gooder,goodest这样的错误了. 再答： 谢谢采纳！