TOEFL Sentence Simplification questions are designed to test a test taker's ability to read and understand complex English sentences, and then to identify the simplest version of the sentence that still conveys the same meaning. The original sentence is usually long and includes multiple clauses, idiomatic expressions, and specialized vocabulary. The simplified sentence is shorter and more straightforward, but still accurate in meaning.
What makes these questions difficult is that test takers need to have a strong understanding of the sentence structure and the context of the sentence. They need to be able to identify the main idea of the sentence and the relationships between the different parts of the sentence. They also need to have a good command of the English language to be able to identify synonyms or alternative ways of expressing the same idea.
What makes these questions relatively easy to answer is that the simplified sentence is usually shorter and more straightforward, so test takers only need to focus on the key information presented in the original sentence. Additionally, these questions usually do not require any outside knowledge or specialized vocabulary.