1
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
先理解核心含义，再看它在真实句子里的用法。
明确无误地；显然
unmistakably; clearly
His explanation was plain wrong.
他的解释显然错了。
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。
She plain forgot to lock the front door last night.
她昨晚显然忘记锁前门了。
It's plain crazy to walk home alone at midnight.
半夜独自走回家简直是疯狂。
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。
Not this
Plain, she didn't understand the question.
Use this
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。
Not this
The data plain shows the trend.
Use this
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。
Not this
That explanation is plane wrong.
Use this
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。
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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)
Use plain naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
平原平坦开阔，一眼望到底，所以 plain 表示“明明白白、毫不含糊”。
语域：informal 常见场景：日常对话、非正式写作 plain 作副词时通常用于非正式场合，表示“明确无误地”，相当于 plainly 的简略说法；在正式写作或严肃语境中，更常见的是使用 plainly。
用作副词时，常置于所修饰的形容词或动词前，表示‘显然、完全’。
Dictionary facts from DICTIONARY_API_DEV, ECDICT, WORDNET.