Hi Dan,

Honestly, I don't understand why "James is Max's supposed Irish ancestor" is right but "Max's grandmother is his supposed Irish ancestor is wrong. After all, both sentences can be simplified to "X is his supposed Irish ancestor".

What I can say though is that an adjective cannot be used to modify another adjective. Only adverbs modify adjectives.

>> "Supposed Irish ancestor" is correct if we want "Supposed" to modify "ancestor". In other words, the sentence would raise doubts as to whether the person is indeed an ancestor.

>> "Supposedly Irish ancestor" would be correct if "Irish" itself was the target of "supposedly". In other words, the sentence would raise doubts as to whether the ancestor is indeed Irish.

As another example, in your initial sentence we have the phrase "a bitterly civil war". "bitterly" is an adverb, so it can only be right if we want to modify the adjective "civil", ie. if we wish to say that the war was "bitterly civil". This doesn't make much sense; it makes more sense to say that the war was "bitter". So we need "a bitter civil war" where the adjective "bitter" doesn't modify "civil" itself (adjective cannot modify adjective), but rather "civil war" or just "war".

Two SCs that test this sort of distinction:

https://www.beatthegmat.com/qn-200-from- ... cityevent=
https://www.beatthegmat.com/adjective-vs ... 22392.html