R.Hello, I have a few questions about comparative grammar.
Just a small quibble - you want to know about the grammar of comparatives. Comparative grammar is a technical term for comparing the grammar of two languages, say English and Cantonese.
When I want to use comparative phrases to describe one's behavior, what is the correct way to change its form?
For instance, should I say "Hold the rail tighter for your safety. " or "Hold the rail more tightly for your safety."?
You can use either in this case.
And how about multisyllabic adjectives ,such as "beautiful"? "More beautifully or more beautiful" ?
The choice of comparatives is 'more beautiful' or 'beautifuller'. The latter does not exist. You have to use 'more beautiful'.
Your use of the adverb 'beautifully' is a different point. The choices here are 'more beautifully' or 'beautifullier". The latter does not exist.
You are confusing two grammar principles - comparison of adjectives and comparison of adverbs. Changing an adjective to an adverb is never a solution to this problem.
loud (adjective); loudly (adverb)
His voice is 'more loud' / 'louder' than mine. ('Louder' is better).
Can you speak 'more loudly' / loudlier? (Only 'more loudly' is right).
As for superlative ones, when people use superlative adjectives to describe things, "the" must be added. How about when used as a verb?
Should I say "Among all the festivals, I like Christmas the best." or "Among all the festivals, I like Christmas best." Which is right?
Either is possible, but generally speaking, using 'the' is more common with the superlative.
Note also, that you have the same problem of choices with the superlatives:
She is the most beautiful / the beautifullest woman in the room. (Only 'most beautiful' is right).
With multisyllablic words, you use 'more <adjective>' or 'more <adverb>' for the comparative, and 'the most <adjective>' or 'the most <adverb>' for superlatives.