Paired comparatives show that as one thing changes, another changes too. The pattern is the + comparative + clause, the + comparative + clause. A clause has a subject and a verb. The two clauses can show a cause and result or simply two changes that happen together.
Build the two clauses
Each half needs its own the and a comparative expression. A comma usually separates the two clauses.
The harder you study, the better your grades get.
The bigger a box is, the more space it needs.
*The: word that begins each half of the pattern.
*Comparative adjective, adverb, or determiner.
*Subject and verb of the clause.
In the first sentence, harder is a comparative adverb that describes study, and better is a comparative adjective that describes grades. In the second, bigger is a comparative adjective and more is a comparative determiner before the noun space. Each half has its own subject and verb: you study / your grades get and a box is / it needs.
Use comparatives with nouns
The comparative can also come before a noun: more books, fewer games, or less time. These words are comparative determiners because they tell us how much or how many. Use fewer with countable plural nouns and less with uncountable nouns.
The more books you have, the more space you need.
The less time you have, the fewer games you can play.
*The: word that begins each half of the pattern.
*Comparative determiner.
*Noun.
*Subject and verb of the clause.
Books and games are countable plural nouns. Space and time are uncountable nouns. That is why the second example uses less time but fewer games.
Avoid common structure errors
Do not replace the second the with than or leave it out. In this pattern, the introduces both changing situations.
The more you read, than more you learn.
The more you read, the more you learn.
The less you sleep, more tired you feel.
The less you sleep, the more tired you feel.
TOEFL strategy
When a sentence begins the + comparative, look for a second the + comparative after the comma. Check that each clause has a subject and verb. If the comparative comes before a noun, check whether the noun is countable or uncountable.