Why couldn't they catch the early train?
They were delayed by______
答案是heavy traffic
為什么不是a heavy traffic
答：樓主的問題其實(shí)很有深度.
名詞的可數(shù)與不可數(shù),很多時(shí)候是相對(duì)的.有不少不可數(shù)名詞,抽象名詞也好,物質(zhì)名詞也好,在一定條件下可以轉(zhuǎn)化為可數(shù)名詞.比如：rain,a heavy rain,樓主印象中有“不可數(shù)名詞前如有一個(gè)形容詞就可以在前面加a"這樣的概念,這是錯(cuò)誤的,也是危險(xiǎn)的.現(xiàn)在知道了,不是所有的.都能加a 的.這就對(duì)了.
查traffic的英語(yǔ)解釋：
Vehicles or pedestrians in transit:
交通：流動(dòng)中的車輛或行人：
heavy traffic on the turnpike; stopped oncoming traffic to let the children cross.
收費(fèi)高速公路上交通繁忙；中斷迎面而來(lái)的車流讓孩子們過街
1 vehicles moving along a road or street 往來(lái)於街道的車輛; 交通
heavy/light traffic 往來(lái)車輛很多[不多]
There's usually a lot of traffic at this time of day.每天在這段時(shí)間往來(lái)車輛都很多.
Traffic was brought to a standstill by the accident.出了事故以後,交通陷於停頓.
London-bound traffic is being diverted via Slough.開往倫敦的車輛現(xiàn)繞道經(jīng)過斯勞.
似乎不能加a,因?yàn)椴皇且淮?不是一個(gè),也不是一陣 的概念.這與a heavy rain不一樣.