There are two ways to form a comparison degree of an adjective in Russian - they are called simple and complex forms. Simple form is when an adjective itself is modified with a certain suffix: красивый (positive) - красивее (comparative) - красивейший (superlative). Complex form is when a certain particle is placed before an adjective, leaving it unmodified: красивый (positive) - более красивый (comparative) - самый красивый (superlative). Now the prefix наи- can be used either with the simple form of superlative degree to make it sound slightly stronger (although bit pompous) - наикрасивейший. Or it can be used to modify a particle for a comparative degree to make it a particle for a superlative degree: более красивый (comparative) - наиболее красивый (superlative) менее красивый (comparative) - наименее красивый (superlative) But it can't be used with simple forms of positive or comparative degrees (наикрасивый/наикрасивее is wrong) and it also can't be used with a simple form of superlative degree of an adjective that have negative prefix не- (I'm not sure that наинекрасивейший is grammatically wrong but it sounds weird).