The most common type of comparison is of things which are non-equivalent. In this case the first element of the comparison consists of an adverb or adjective inflected in -er or -est or preceded by more, most, less. least; the second element is introduced by than or a preposition such as of or in. For example:
John is bigger than Bill.
John is the biggest in the class/of the three.
John is more/less intellectual than Bill.
John is the most/the least intellectual boy in the class/of the three.
A second type of comparison is that of equivalent items. Here both elements of the comparison are introduced by as, e.g.:
John is as big/intellectual as Bill.
Other types of comparison are based on relationships of what Quirk et al. (1972:§11.62-63) call "excess" and "sufficiency". Comparisons of excess involve too and an infinitive clause:
John is too short to make the basketball team.
Comparisons of sufficiency involve enough and an infinitive clause:
John is tall enough to make the team.
Using negation, sentences with these comparative structures can be paraphrased in terms of each other, e.g.:
John is too short/not tall enough to make the team.
John is tall enough/not too shortto make the team.
The relationship of excess can also be expressed by so (before adverbs and adjectives) or such (before nouns) and a that clause, e.g.:
John is so tall/such a tall person that he can touch the backboard.
This structure cannot express sufficiency, however, unless sufficiency also implies excess or an extreme degree. Thus it would be odd to say
John is so short that he can't make the team.
because this would imply not only that he is too short to make the team but also that he is short by some general standard. Similarly, to say
John is so short that he can't touch the backboard.
would give the false impression that being able to touch the backboard is generally expected (as perhaps it might be among professional basketball players), whereas in fact only very tall people can do so. Compare:
John is so short he can't reach the light switch.
This is all right as long as John is an adult; if he is a growing child it would imply that he is very short for his age.
Errors involving comparison of non-equivalent items are discussed in 7.1 and 7.4. Comparison of equivalent items is treated in 7.2. Comparisons with enough/too and such/so are discussed in 7.3 and 7.5.