Pure substances by definition are a single type of building block.
Let us take the example of the popular building blocks for kids - Legos. We can consider the idea of a single red block as an element. Now if we stack 10 red blocks one atop the other we can call that structure a compound as it is made from 10 unit pieces. Really? Can we really? For those of you who caught it congratulations.
A compound is defined as a combination of two elements in a set ratio. Now say I made a structure of 5 red then 5 blue on top of that - that is a compound. You may be thinking "I understand why an element is a pure substance - it's a single building block. But why is a compound considered when a mixture isn't?"
The thing that is special with a compound is that the ratio of red to blue blocks will always be the same. If I wanted to provide an example of how they can be building blocks I would say stack 5 red then 5 blue on top then 5 red then 5 blue keep going with this pattern another three sets. You can see that the ratio of red to blue is still 1:1 as it was in the original set and therefore acts as a building block.
If we had a mixture of red and blue we could say 3 red and then 7 blue then 6 red on top of that and 4 blue at the top of the pile - now keep generating random numbers to add for three more sets - the ratio could be absolutely anything. As you can see the pattern doesn't have to repeat so it doesn't act as a building block and therefore isn't a pure substance.
Edit: I recently read over my post and realized that i had made a mistake. It must be made clear that elements and compounds cannot be broken down by PHYSICAL means, they can still be broken down chemically. eg. A chemical reaction.