Expressing location with "zai... shang / xia / li"
You can use 在 (zài) to express location, but this article will explain how to use 在 to express location in relation to another object. This way, you can describe if something is "on the table" or "in the room".
To show where an object is in relation to another object, you can use 在 and a direction complement.
在 + Location + 上/下/里/旁边/etc
Notice that the "preposition" comes after the location that it modifies. That is, in English we say "on the table," but in Chinese, it is more like "at the table, on." This can be somewhat confusing, but don't worry. Once you start using this construction, it gets easy really quickly.
Some prepositions can be paired with more abstract concepts to form idiomatic phrases.
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.
在 + Abstract Concepts + 上 + Comment
在 + 下
This construction can be translated (rather directly) as "under" and expresses conditions that affected the comment that comes after.
在 + Abstract Concept + 下 + Result/Comment
在 + 方面
This construction is very similar to the "在...上" above. It translates to "in relation to" or "on the topic of".