How do I decide between the prepositions "in" and "on"?

Question

I'm wondering about and still confused over when to use on and in, for example like the sentence below:

1. "I like to sunbathe in the sun." In my study I figured out that when using in you the thing that you are talking about should be enclosed, or in the center, or inside, right?

   How did it become in when you are not in the sun?

2. In a conversation of referring something I have always seen a lot of people using on, like "You are on your own." Why is it not in your own? I'm so confused!

I already know about the prepostions of time and place and when to use them but in these conversations I'm still rather confused about what I'm going to need.

Answer 1

in refers to immersion or being surrounded or enclosed.

You can stand in the rain.

You can stand in the sun(light).

You can stand in the river.

You can sit in the grass.

on refers to a basis or foundation.

You can sit on the grass.

You can sit on the lawn. (But you can't sit in the lawn).

You can stand on the ground.

You can stand on principle.

You can stand on your own two feet.

You can stand on your own.

You can stand on (the ice of) the frozen river.