Old English:

The wedding-Guest sat on a stone,

He cannot chuse but hear,

And thus he spake on that ancient man,

The bright-eyed Mariner

Normal:

The wedding-Guest sat on a stone,

Forced to listen and hear,

And then spoke to that ancient man,

The bright-eyed Mariner.

I think this paragraph is saying that the man/ woman is meant to be at a wedding but instead the Mariner forced him to sit down on a stone and listen to him.