

– It is the most cosmopolitan city in America In these examples, you would not use a superlative suffix. In any other superlative adjectives that are longer than a single syllable, simply add either ‘most’ or ‘least’ to the positive form of the word.

As usual in English, these are not all always true, but a good basis for using superlative adjectives correctly: There are a number of standard rules to use the superlative suffix.
