Flat 多數(shù)情況下會用做形容詞來表示“平坦的......”。

Plain 多數(shù)會用為名詞來表示“平地”。用作名詞的時候：Flat - (n) the flat part of something.一件東西的扁平部分Plain - (n) a large area of flat land with few trees.大面積的平地（樹木稀疏）用作形容詞的時候：Flat - (adj.) smooth and even; without marked lumps or indentations.平坦而均勻; 沒有明顯的凹凸。Plain - 1. (adj.) not decorated or elaborate; simple or ordinary in character. 2. easy to perceive or understand; clear.表面光光的，沒有任何東西覆蓋普通及簡單明了例子：That flat wall is really plain.那堵平平的墻光禿禿的。This garden looks too plain, and the ground isn't flat as well.這個花園看上去太普通了，而且地也沒鋪平。如果英語水平還不錯，可以參考這個網(wǎng)站：Plain vs Flat - What's the difference