Flat 多数情况下会用做形容词来表示“平坦的......”。

Plain 多数会用为名词来表示“平地”。用作名词的时候：Flat - (n) the flat part of something.一件东西的扁平部分Plain - (n) a large area of flat land with few trees.大面积的平地（树木稀疏）用作形容词的时候：Flat - (adj.) smooth and even; without marked lumps or indentations.平坦而均匀; 没有明显的凹凸。Plain - 1. (adj.) not decorated or elaborate; simple or ordinary in character. 2. easy to perceive or understand; clear.表面光光的，没有任何东西覆盖普通及简单明了例子：That flat wall is really plain.那堵平平的墙光秃秃的。This garden looks too plain, and the ground isn't flat as well.这个花园看上去太普通了，而且地也没铺平。如果英语水平还不错，可以参考这个网站：Plain vs Flat - What's the difference