便宜

便 often reads biàn in 方便, so learners guess biàn yí for 便宜. But cheap is pián yi, with a second-tone 便 and a soft, toneless 宜. If you say biàn yí, you are not saying cheap. The old word 便宜 biàn yí means convenience or advantage, but modern daily Chinese uses 便宜 pián yi for cheap.

便宜 is fine for things you are buying yourself or for budget goods. When you comment on a friend's new jacket, 很便宜 can hint it is low quality. 不贵 means not expensive, which sounds neutral and polite. So if the jacket looks nice, say 不贵 or 挺好看的, not 很便宜.

是 links two nouns, like A is B. It does not sit before a describing word like 便宜. To say something is cheap, use 很便宜 or just 便宜. 很 often makes the sentence sound complete, but it does not always mean very. The story sentence uses this pattern with no 是. “大苹果很便宜，小苹果很贵。” (The big apples were very cheap, and the small apples were very expensive.) ── [《很贵的苹果》](/read/buying-expensive-apples#s3)

English cheap covers both low price and stingy. Chinese 便宜 only covers low price. If you call a person 便宜, it sounds insulting and often suggests they are sexually cheap. For stingy, use 小气 or 抠门. So 我朋友很便宜 is a bad translation of my friend is cheap.