Is "This flower is beautifuller than that one" correct?

Why

Sohan's rule works perfectly for short adjectives — "tall" becomes "taller", "fast" becomes "faster" — so it's a fair pattern to try extending. English actually splits comparisons into two different methods depending on the adjective's LENGTH. Short, usually one-syllable adjectives add "-er"/"-est" directly. Longer adjectives, especially those with three or more syllables like "beautiful" (beau-ti-ful), instead use a separate helper word, "more"/"most", placed in front of the unchanged adjective — "more beautiful", "most beautiful" — rather than being stretched out with a clunky ending. "Beautifuller" breaks this pattern and sounds awkward to a fluent ear precisely because long adjectives were never meant to carry that ending. Test: try adding "-er" to other long adjectives — "interestinger", "difficulter" — and notice they all sound equally wrong, while "more interesting" and "more difficult" sound natural, confirming this is a length-based pattern, not a one-off exception. His sister is right: longer adjectives use "more"/"most" instead of "-er"/"-est", so it's "more beautiful", never "beautifuller".