be + plain + adjective
plain 作副词为非正式用法，等同于 plainly，常置于形容词前加强语气。
His explanation was plain wrong.
他的解释显然错了。
明确无误地；显然
plainest — superlative of plain
明确无误地(plain 常非正式地用作 plainly)；清楚地；显然地；[计] 无格式
unmistakably; clearly
非正式用法，相当于 plainly
明确无误地(plain 常非正式地用作 plainly)
unmistakably (`plain' is often used informally for `plainly')
This map is plain wrong about the highway exit.
这张地图关于高速公路出口的信息显然是错的。
用法提示：副词 plain 放在形容词 wrong 前，表示“显然、完全地”。
She plain forgot to lock the front door last night.
她昨晚显然忘记锁前门了。
用法提示：这里 plain 作副词修饰动词 forgot，是口语用法，表示“完全、确实”。
It's plain crazy to walk home alone at midnight.
半夜独自走回家简直是疯狂。
用法提示：plain 是副词，修饰形容词 crazy，表示“显然、毫无疑问地”。
plain 作副词为非正式用法，等同于 plainly，常置于形容词前加强语气。
His explanation was plain wrong.
他的解释显然错了。
此结构中 plain 修饰其后的形容词，而非名词。
That's a plain silly question.
那是一个显然很蠢的问题。
grammar · medium
易错句子Plain, she didn't understand the question.
正确表达Plainly, she didn't understand the question.
plain 作副词表示“显然”时不能放在句首作评注性状语；句首应使用标准副词 plainly。这是受汉语“显然，……”句式影响而误用。
grammar · medium
易错句子The data plain shows the trend.
正确表达The data plainly shows the trend.
plain 虽然可作副词，但一般只用于形容词前或动词后（如 plain wrong、tell sb plain），不能放在行为动词前作状语；动词前表示“清楚地/显然地”应用 plainly。
grammar · medium
易错句子That explanation is plane wrong.
正确表达That explanation is plain wrong.
plane（飞机/平面）与 plain（显然/清楚的）同音，此处是副词 plain 修饰形容词 wrong，表示“明显错误的”，应写作 plain 而非 plane。
完全错误
It's just plain wrong to lie.
撒谎完全是错误的。
明显愚蠢的
That idea is plain silly.
那个想法明显很愚蠢。
十足疯狂
Climbing that cliff in this weather is plain crazy.
在这种天气爬那座悬崖真是太疯狂了。
非常懒惰
He didn't finish the report; he's just plain lazy.
他没完成报告；他就是太懒了。
筋疲力尽
After the long trip, we were plain tired.
长途旅行之后，我们累坏了。
明明白白；说白了
The problem is, plain and simple, a lack of funding.
问题说白了就是资金不足。
用简单明了的话；直白地
Can you explain the rules in plain English?
你能用简单明了的话解释一下这些规则吗？
显而易见；一清二楚
Her disappointment was as plain as day.
她的失望一清二楚。
发长 /eɪ/ 音 (如 rain, wait)
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Dictionary facts from DICTIONARY_API_DEV, ECDICT, WORDNET.