When I looked at differential geometry, I'd already seen Hestene's Geometric Algebra (a Clifford Algebra) and I was confused that you couldn't combine n-forms with m-forms in DG. The link is (I believe!) that Clifford Algebras are _graded_ algebras, whereas DG's n-forms are not graded, so in DG you can't combine forms of different degree arbitrarily.

For a better explanation I can recommend the accepted answer here:

https://math.stackexchange.com/questions/1991814/whats-the-d...