You should use the comparative form of an adjective or adverb to compare exactly two things. You can form the comparative by adding the suffix "-er" to the modifier (for some short words) or by using the word "more" with the modifier:
Of the two designs, the architect is convinced that the city will select the more experimental one. (comparing two designs)
Now that it is March, the days are getting longer. (longer now than before)
You should use the superlative form to compare three or more things. You can form the superlative by adding the suffix "-est" to the modifier (for some short words) or by using the word "most" with the modifier:
This is definitely the smartest, wittiest, most imaginative comic strip I have ever seen. (implying that I have seen more than two)
Note: if you are not certain, you should check a dictionary to see which words take use "more" and "most" and which words take the suffixes "-er" and "-est."
Common Problems with the Comparative and Superlative
There are certain modifiers which you cannot logically use in the comparative and superlative forms. Adjectives like "perfect" and "unique," for instance, express absolute conditions and do not allow for degrees of comparison. Something cannot be more perfect than another thing: it is either perfect or not perfect.
You should also avoid using a double comparison -- that is, using both a suffix and an adverb to indicate the comparative or superlative:
[WRONG] I am convinced that my poodle is more smarter than your dachshund.
[WRONG] Laurel and Hardy are the most funniest slapstick comedians in film history.
[RIGHT] I am convinced that my poodle is smarter than your dachshund.
[RIGHT] Laurel and Hardy are the funniest slapstick comedians in film history.
Similarly, although the double negative -- the use of two negative words together for a single negative idea -- is common in speech and has a long history in the English language, you should avoid using it in formal writing:
[WRONG] We decided there wasn't no point in pursuing our research further.
[WRONG] I can't get no satisfaction.
[RIGHT] We decided there wasn't any point in pursuing our research further. OR We decided there was no point in pursuing our research further.
[RIGHT] I can't get any satisfaction. OR I can get no satisfaction.
Double negatives involving "not" and "no" are fairly easy to spot and fix. However, some other adverbs -- for example, "hardly," "scarcely," "barely" -- imply the negative, and you should not use them with another negative:
[WRONG] Even though he has lived in Toronto for four years, he does not have hardly any friends there.
[RIGHT] Even though he has lived in Toronto for four years, he has hardly any friends there. OR Even though he has lived in Toronto for four years, he does not have many friends there.