rain 是不可数名词,为什么前面可以带不定冠词(a rain),而且还有复数形式(rains)呢?
rain 作“雨”解时是不可数名词,不能带不定冠词,而且也没有复数形式。
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Rain is much needed. 非常需要雨。
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Rain fell at intervals throughout night. 雨断断续续地下了一整夜。
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We have had too much rain this summer. 今年夏天雨下得太多。
如果带有不定冠词,则其意义就会发生变化。或者是指具体的“一阵雨”“一场雨”（此时常常带有定语），或者是用作比喻义：“雨点般的东西”。
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A cold rain was drizzling down. 正下着寒冷的蒙蒙细雨。
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There was a light rain falling. 下了一场小雨。
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There was a heavy rain last night. (OSD) 昨夜下了一场大雨。
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You can't go home in this rain. (EED) 你不能在这场雨中回家。
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A rain of arrows fell on the soldiers. 雨点般的箭落在战士们身上。
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a rain of tears (bullets, questions) 雨点般的泪水(子弹、问题)
rains 则是指“大阵雨”“雨季”，尤其指热带地区的“雨季”：
The rains have started early this year. (LDCE) 今年雨季来得早。
Take an umbrella with you in case of _.
What ____! We were wet all over.
Toward evening, ____ came, which made things even worse.
There ____ lots of ____ in the north this year.