In cases that the conjunctive II sounds strange or is simply unknown to the speaker we come to - what the author would like to call - rule number three and a half. Because the fact that something is unknown or sounds weird is very much subjective. This what might feel strange to me or what I just don't know might be cristal clear to others.

Er sagt: "Wir fahren nach Berlin."

Transformation according to rule number one: Er sagt, dass sie nach Berlin fahren. = conjunctive is identical to present indicative Transformation according to rule number two. Er sagt, dass sie nach Berlin führen. = conjunctive II is NOT identical to imperfect, but to many people it sounds weird. Now we take rule number three 1/2 and transform into conditional. Er sagt, dass sie nach Berlin fahren würden.

Er sagt: "Wir trinken ein Bier."

Transformation according to rule number one: Er sagt, dass sie ein Bier trinken. = conjunctive is identical to present indicative Transformation according to rule number two Er sagt, dass sie Bier tränken. = This sounds to many people weird. Transformation according to rule number three 1/2 Er sagt, dass sie ein Bier trinken würden. = conditional