What is "comparatives and superlatives"?

They are forms of comparison of adjectives.
For example, we can say that a car is big, that it is bigger than another, or that it is the biggest car of all. (We use the suffixes -er and –est)
Alternately, we can say that a car is big, that it is more big than another, or that it is the most big car of all. (We use the the words more and most.)
By either approach, English adjectives therefore have positive forms (big), comparative forms (bigger or more big), and superlative forms (biggest or the most big.)
I have tried my best to help you.

comparatives and superlatives比較詞和最高級
1. Eliza is the oldest of my two daughters.
2. Having his choice of two offices, Melvin opted for the one closest to the elevator.
3. Of the three cities the committee considered for the conference next fall, Charleston has more restaurants and sightseeing opportunities.
4. The tomato plants in our yard this year are more fruitful than any we have ever grown.
Not one of these sentences is correct.
The first two sentences incorrectly use the superlatives oldest and closest to compare only two daughters and two offices. Instead, each sentence should have used the comparative degree:
Eliza is the OLDER of my TWO daughters.
Having his choice of TWO offices, Melvin opted for the one CLOSER to the elevator.
Sentence 3 compares three cities and thus should have used the superlative degree:
Of the THREE cities the committee considered for the conference next fall, Charleston has THE MOST restaurants and sightseeing opportunities.
The sentence could have used the comparative degree if it were written so that each of the other two cities was being compared separately to Charleston:
Of the three cities the committee considered for the conference next fall, Charleston has MORE restaurants and sightseeing opportunities than EITHER of the other two.
Remember that when we compare two people, places, or things, we should use the comparative degree (more or -er):
She is the MORE studious of the two children.
Our office complex is the TALLER of the two on our block.
But when we compare three or more people, places, or things, we should use the superlative degree (most or -est):
She is the MOST studious child in her class.
Our office complex is the TALLEST one in town.
Sentence 4 is incorrect because it is illogical. If the tomato plants in our yard this year are more fruitful than ANY tomato plants we have EVER grown, then they are more fruitful even than themselves. The correct way to make such a statement is to say that they are "more fruitful than any OTHER tomato plants we have ever grown." We use the word other because what we mean is to compare this year's crop to the crops of each earlier year--one year at a time. Because we are, in other words, actually comparing only two years' crops, we should use the comparative "more fruitful than. . . ."
We could, however, rephrase the sentence in a way that makes the superlative appropriate:
The tomato plants in our yard this year are the MOST FRUITFUL we have ever grown.
Here the superlative degree makes sense because we are comparing this year's crop to the crops from ALL earlier years; that is, we are simultaneously comparing more than two crops.
TEST YOURSELF
Can you spot problems with any of the following comparisons? (The answers are below.)
1. The highway connecting Bloomingdale to Mooresville is more frequently traveled than any road in California.
2. Of all the annual reports we have read in the past decade, James Little's was more concise.
3. Three employees made suggestions for improving the company's health care benefits, but Susan Miller's plan was the most practical and cost effective.
4. Faced with two investment portfolio options, most employees chose the more aggressive one because it has performed best during the past five years.
5. We examined two vacation packages and decided that the trip to Disney World was the most affordable.
ANSWERS
1. more frequently traveled than any OTHER road in California (or ". . . is the MOST frequently traveled road in California.")
2. the MOST concise
3. correct
4. it has performed BETTER during the past five years (MORE aggressive is correct)
5. Disney World was MORE affordable