It is always smart, smarter, smartest. Clever, cleverer, cleverest is proper English. However it is accepted to use more clever and most clever. This is because of the rule that Mohammed mentioned. English is VERY difficult to learn because of the numerous exceptions it has! With the exception of the irregular ones, such as "good, better, best, bad, worse, worst", the general rule for one and three or more syllabi holds. It is NOT always valid for words with two syallabi. The following are some exceptions that come to my mind! Happy, Happier Happiest! Friendly, Friendlier, Friendliest. Easy, Easier, Easiest and so are crazy, dirty, funny, squeaky. For more info see http://grammar.quickanddirtytips.com/better-versus-best.aspx "Most" is already the superlative. However, it is acceptable to use "mostest" even though it is technically wrong! For a more interesting article, see http://grammar.quickanddirtytips.com/comparatives-versus-superlatives.aspx

the adjectives are either one syllable ex:( smart ,tall,short big , ..etc ) or more than one ex:(beautiful,clever, strong ,...etc ) . if the adjective consists of one syllable we add ( er ) . smarter is correct . if it consists of more than one syllable we add ( more ) . more clever is correct . thanks :)

smarter and cleverer

Smarter and more clever are correct.

Thank you Professor B , for your nice answer.I'm still learning and your notice is very precious . Thanks again :)