因为只能是单音节形容词加er，而crowded为多音节，比较级为more crowded，中文意思是更拥挤的
例如
你见过比这更拥挤的超市吗Have you ever seen a supermarket more crowded than this one?
杰夫：嗯，纽约更有趣，但是那儿的物价也更高，交通更拥挤。
JEFF: Well， New York is more interesting， but it's also more expensive， more crowded.
crowded比较级为什么不加er
crowded比较级是：more crowded. 最高级是most crowded，根据比较级和最高级的变化规律表明，双音节或者是多音节单词在变化的时候需要在单词前加比较级more更高级most
双音节、多音节形容词，在单词前面加上 “more” “most” 构成比较级、最高级：
afraid（害怕的）—more afraid—most afraidbeautiful（美丽的）—more beautiful—most beautiful careful（仔细的）—more careful—most carefulcheerful（开心的）—more cheerful—most cheerful crowded（拥挤的）—more crowded—most crowdeddangerous（危险的）—more dangerous—most dangerous delicious（美味的）—more delicious—most delicious difficult（困难的）—more difficult—most difficultexciting（令人兴奋的）—more exciting—most exciting expensive（昂贵的）—more expensive—most expensive famous（著名的）—more famous—most famousimportant（重要的）—more important—most importantinteresting（有趣的）—more interesting—most interesting polite（有礼貌的）—more polite—most polite terrible（可怕的）—more terrible—most terrible tired（累的）—more tired—most tired
