What keys can I modulate to when harmonizing a Bach chorale?

There are 5 keys which a Bach chorale could modulate to*. Two of these are the dominant and the subdominant (based on the 5th and 4th degree of the home key respectively). If the home key is major then the chorale could modulate to the relative minor as well as to the relative minor of the dominant or the subdominant. If the home key is minor then the opposite applies (i.e. it can modulate to the relative major of the home key, dominant, or subdominant).For example, if the home key of a chorale is D major, it could modulate to: B minor (the relative minor), A major (the dominant), F sharp minor (the relative minor of the dominant), G major (the subdominant) and E minor (the relative minor of the subdominant). A chorale in B minor could modulate to all of these keys as well as to D major.*Bach does sometimes use keys other than these but you would not encounter this in your A level chorale as it is rare and unusual.