Comparison words require balance too. When you use than, as ... as, or the same as, the two sides should name or describe the same kind of thing. Compare a noun with a noun, an action with an action, and a quality with a quality.
Compare noun with noun
The population of Mexico City is larger than Monterrey.
The population of Mexico City is larger than the population of Monterrey.
*Comparative adjective.
*Comparison word.
*Noun phrase.
The first sentence compares a population with a city, which is not logical.
Compare actions with actions
Studying with classmates is as effective as to study alone.
Studying with classmates is as effective as studying alone.
To study with classmates is as effective as to study alone.
*Gerund phrase.
*Infinitive phrase.
*Adjective
*Comparison word.
The correct sentence compares one activity with another activity. Both activities must be equal. Either gerund phrase + gerund phrase or infinitive phrase + infinitive phrase.
Compare clauses with clauses
The new library provides more computers than the old library does.
The new library has more computers than the old library.
*Subject.
*verb.
*Comparison.
*Comparison word.
In the first example, the old library does replaces the old library provides computers. This avoids repeating the full verb phrase.
TOEFL strategy
Ask, “What exactly am I comparing?” If the two sides do not match, the sentence is probably incorrect. In particular, watch for sentences that compare a quantity with a place, a person with an action, or a noun with a clause.