How are the differential forms in $S^1$?

Question

I'm really confused about differential forms in Manifolds.

I know that the usual differential forms in $\mathbb{R}^n$ be like for example

$dx_1+dx_2+dx_3+...$ ($1-form$)

$dx_1\wedge dx_3+dx_2\wedge dx_4+dx_3\wedge dx_4+...$ ($2-form$) , etc.

But how looks a differential form in $S^1$?