Hi,
"We know it's a noun because we cannot add "er" like we can with adjectives"
I'm curious if it's correct or not. If it's true, it can surely make things simpler.
No, the -er method suggested above is not a good idea. With only some adjectives, we add -er to make the comparative form. eg small - this object is smaller than that object. In simple terms, -er is used with adjectives of one syllable and adjectives that end in 'y' (eg happy). However, with other adjectives, we use the word 'more', eg more interesting.
Best wishes, Clive