_____ more slowly,please.用 say,speak,tell还是 talk

楼上两位都不对.副词也有比较级,比如这里的slowly
但是slowly是从slow加词尾ly变来的,这样的副词比较级和最高级只能是more,most,不能把y变成ier
所以应该是more slowly
楼主所说那两个规律是对的,但前一个用于简单形容词,后一个没问题.
另外如果形容词或副词音节比较多,也会用more,most来变,至于多少算多,似乎没有定论.比如more complex
还有动词的现在分词和过去分词引申而来的形容词,只能用more,most,比如more interesting,more excited
还有一些词就是习惯用法了,比如more often,more common,没有为什么

Slow和slowly当副词时同义,但slow比slowly语气强.
I told the driver to go slow(er).
Drive slowly round these bends in the road.

slow的比较级是slower不是slowly
slowly是副词
你说的more slowly 是slowly的比较级.Please speak more slowly.请再慢点说.

slower与more slowly都是比较级,但一个是形容词,一个是副词
slower是形容词,用来修饰名词,放在名词前,
如slower speed（慢一些的速度）
more slowly是副词,用来形容动词,放在动词后,
如act more slowly(行动得更慢)

than表明比较级
由形容词+ly变来的副词变比较级：more+副词

slow只有1个音节
slowly有2个啊

Could you please drive more slowly?There are many differences between America and Endland .Do not leave your homework at home.Keeping n_____ is our duty.

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.

没有slowlier这种说法的,形容词的比较级是加more体现出来的.
副词没有比较级.

slowly是多音节词,多音节形容词、副词比较级在前面加more

对
slowly是副词
副词比较级前加more
除early以外

一般来说双音节的形容词变比较级是前面加more
而slowly可能是特殊规定的吧.

early 既是形容词也是副词,形容词的比较级一般都是加er的
而slowly只是作为副词存在,副词的比较级一般都在前面加more
还有一个原因就是early的形容词形式还是early
但是slowly却是slow

Why do you always follow me?
Please speak more slowly I can't follow you.