In the West is being in the western region of an area. I am in the Western United States. I am in West Philadelphia. I am in the West. On the west is used in rare circumstances such as the following: On the west coast, I go surfing. On the west side of town, there are many cars. On the western frontier, people made their new lives. Famous Book title: All Quiet on the Western Front You can say "I am in the west." You would never just say "I am on the west." There is not grammatical rule for using them in phrases. It is just memorizing. Hope that helps.

"In the West" is straightforward. It means in the western part of whatever geogphaphic entity you're talking about. When used alone, it usually refers to Europe and the Americas, the seats of Western civilization. In the USA, for instance, it refers to the western part of the country. It likely means something different in other countries. "On the West"? No idea. Maybe some context would help.

To be 'in' the West is being in the Western region of a country (this is the correct one of the two). However, being "on" the West would be standing on the ground in the Western region (this is never said).

"in the west" can be used on its own. "on the west" needs to be followed by a noun: on the west side, on the west coast.