this one is narrower or this one is the narrowest more narrow is sweet but not the best English

The words "more" and "most" are used for adjectives with 3 or more syllables. (eg: beautiful, more beautiful, most beautiful) The "-er" and "-est" are used for adjectives with 1 syllable. (eg: fat, fatter, fattest) Daniel is right.

both "most narrow" or "narrowest" can be used in standard english. "what is the most narrow river in the world?" "what is the narrowest river i the world?" I think only if we are describing one thing in a group of three or more things, wwe are more likely to use "narrowest." ex: "between Nile, Amazon and Mississippi rivers, Mississippi is the narrowest." :)

The formal rule I was taught in primary school is: Adjectives and adverbs of two syllables or less recieve the -er, -est suffixes. Adjectives and adverbs of three syllables or more recieve the more, most modifiers. This rule is usually, but not always, observed in informal speech. Also, there are a few irregular constructions, such as: good, better, best bad, worse, worst tl;dr: narrowest

either "the most narrow" or "the narrowest" is sounds OK to me. The canyon was at it's narrowest point, when the boat overturned. The canyon was at it's most narrow when the boat capsized. Both work equally, and do not sound awkwardly phrased. A. :)