You probably already know how to express location with "在". In this article, we'll go more in depth as to how you can make idiomatic phrases using that construction. This kind of form is actually similar to some that we have in English, so it won't be too difficult.

Expressing "on the topic of" with "在...上"

This construction can be translated as "on the topic of" or "in regards to". It can be used to comment on an idea or concept. The examples below will help.

Examples
Zài yú fǎ shànɡ, wǒ zhī dào wǒ hái yǒu hěn duō wèn tí .
在 语法 上，我 知道 我 还 有 很多 问题。
In regards to grammar, I know I still have a lot of questions.

Expressing "under" with "在...下"

This construction can be translated (rather directly) as "under" and expresses conditions that affected the comment that comes after.

Examples

Zài tài tɑi de zhī chí xià ， wǒ zì jǐ kāi le yī ɡè ɡōnɡ sī.
在 太太 的 支持 下，我 自己 开 了 一 个 公司。
With the support of my wife, I was able to start my own business.

Expressing "on the topic of" with "在...方面"

This construction is very similar to the "在...上" above. It translates to "in relation to" or "on the topic of".
Examples

Zài wǎnɡ yóu fānɡ miàn, tā shì zhuān jiā.
在 网游 方面，他 是 专家。
On the topic of online games, he is an expert.