How to understand that an empty set is not a proper subset of any set? I know the meaning, but how to understand it

How to understand that an empty set is not a proper subset of any set? I know the meaning, but how to understand it

An empty set is a subset of any set
An empty set is a proper subset of any nonempty set
So it is true that an empty set is not a proper subset of any set

Given a = {a, B, C}, then the number of proper subsets of a is______ ．

There are three elements a, B and C in the set a, which are substituted into the formula: 23-1 = 7, then the true subsets of the set a are: {a}, {B}, {C}, {a, B}, {B, C}, {a, C},}, a total of seven. So the answer is: 7

{a} Is a subset of M, and M is a proper subset of {a, B, C, D}. How many sets are there in total

First of all, m must have element a. the problem is to find the number of subsets of {B, C, D} because m is a proper subset, so there are 7 subsets