"More harmful" and "most harmful." We would not use the -er and -est suffixes on the -ful which is already a suffix.
"harmful" doesn't have a comparative and superlative form. Instead, you would say "more harmful" or "most harmful."
It is more dangerous and then most dangerous.
more harmful
more dangerous
more dangerous
More harmful and most harmful
unhealthier
comparative - merrier superlative - merriest
comparative = tenderer superlative = tenderest
jollier [comparative] jolliest [superlative]
The comparative is later, and the superlative is latest
The comparative and superlative are fairer and fairest.
harmful
more harmful, most harmful
what is the comparative and superlative of glad
What is the comparative and superlative of Often
comparative - merrier superlative - merriest
The comparative is greater and the superlative is greatest.
comparative - rockier superlative - rockiest
comparative is calmer superlative is calmest
The comparative is higher. The superlative is highest.
lovelier [comparative] loveliest [superlative]
comparative = tenderer superlative = tenderest
The comparative form of "grumpy" is "grumpier," and the superlative form is "grumpiest."