平时 (píngshí) and 平常 (píngcháng) look very similar (after all, they do start with 平!), and they have similar meanings of "often," but they have pretty different grammatical uses.
平常 is used as noun
平常 can be used as a noun. It means "in ordinary times:, as in "ordinary times I usually meet with my friends after school."
Structure
Examples
- 我 平常 不 和 朋友 见面。 I ordinarily don't meet friends.
- 你 平常 用 QQ 聊天 吗？ Do you usually use QQ to chat?
平时 is used as noun
When 平时 is used as a noun it is the same as 平常 when it is used as a noun.
Structure
Examples
- 我们 平时 都 不 在 家 吃饭，我们 在 外面 吃。 Most of the time, we don't eat in the house. We go out to eat.
- 你 平时 会 看 新闻 吗？ Will you watch the news often?
- 我 平时 不 买 这 种 很 便宜 但是 质量 很 差 的 东西。 I don't usually buy this kind of cheap, inferior stuff.
平常 as an Adjective
When 平常 is used as an adjective, it expresses the meaning of "common." 平时 cannot be used this way.
Structure
Examples
- 他 有 一 份 很 平常 的 工作，过 着 很 平常 的 生活。 He has a common job and lives a common lifestyle.
- 这 是 一 种 很 平常 的 玻璃，不 是 什么 钻石。 This is just common glass. It's not a diamond.
Example dialog
- A: 你 平时/平常 喜欢 研究 玉 对 吗？ You often like to study jade, right?
- B: 对 啊。 Correct.
- A: 你 帮 我 看 一下 这 块 玉 怎么 样？ Could you tell me about this piece of jade?
- B: 是 一 块 很 平常 的 玉。 It's a common piece of jade.
Sources and further reading
Books
- 现代汉语八百词（增订本） (“平常”p. 1051)
- 现代汉语八百词（增订本） (“平时”p. 1053)